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Host Pathogen Interaction

Table of Contents

Host Pathogen Interaction

  • The host–pathogen interaction is described as the molecular, cellular, organismal, or population-level sustenance of bacteria or viruses within host organisms.
  • This phrase is widely used to describe to bacteria that cause disease, however they may not infect all hosts.
  • As a result, the concept has been expanded to include how infections persist within their host, regardless of whether they cause disease.
  • On the molecular and cellular level, microorganisms can infect the host and divide rapidly, producing disease by establishing an imbalance in the body’s homeostasis or by secreting toxins that result in the appearance of symptoms.
  • Viruses can also infect the host with pathogenic DNA, which can interfere with normal cellular activities (translation, transcription, etc.), protein folding, or immune evasion.

What is Infection?

  • Infection is the infiltration of tissues by pathogens, their growth, and the responsiveness of host tissues to the infectious agent and toxins produced by it.
  • There are five stages of an infection: incubation, prodrome, sickness, decline, and recovery.
  • An infectious disease, often known as a communicable disease or transmissible disease, is a condition caused by an infection.
  • Infections can be caused by a variety of pathogens, bacteria and viruses predominating.
  • Using their immune systems, hosts can combat infections. In response to infections, mammalian hosts exhibit an innate response, which frequently involves inflammation, followed by an adaptive response.
  • Infections are treated with antibiotics, antivirals, antifungals, antiprotozoals, and antihelminthics, among others.
  • In 2013, infectious diseases caused 9.2 million fatalities, or almost 17 percent of all deaths. Infectious disease is the branch of medicine that focuses on infectious diseases.

Types of Infection

Infectious agents (pathogens) that cause infections include:

  1. Bacteria (e.g. Mycobacterium TB, Staphylococcus aureus, Escherichia coli, Clostridium botulinum, and Salmonella spp) (e.g. Mycobacterium tuberculosis, Staphylococcus aureus, Escherichia coli, Clostridium botulinum, and Salmonella spp.)
  2. Viruses and associated agents, including viroids. HIV, Rhinovirus, Lyssaviruses such as the Rabies virus, Ebolavirus, and Severe acute respiratory syndrome coronavirus 2 are examples.
  3. Fungi are further subdivided into:
    1. Ascomycota, which includes yeasts such as Candida (the most prevalent fungal infection), filamentous fungus such as Aspergillus, Pneumocystis species, and dermatophytes, are a group of organisms that cause infection of the skin and other superficial structures in humans.
    2. Among the Basidiomycota is the human-pathogenic genus Cryptococcus.
  4. Parasites are often classified as:
    1. Unicellular organisms (e.g. malaria, Toxoplasma, Babesia)
    2. Macroparasites (worms or helminths) such as parasitic roundworms and pinworms, tapeworms (cestodes), and flukes (trematodes, such as schistosomes). Helminth-caused diseases are sometimes referred to as infestations, and occasionally as infections.
  5. Ticks, mites, fleas, and lice can also cause human sickness, which are conceptually similar to infections; nevertheless, the invasion of a human or animal body by these macroparasites is typically referred to as an infestation.
  6. Prions (while they do not release toxins) (although they do not secrete toxins)

What is Invasion?

  • The invasion of a host by a pathogen may be facilitated by the creation of bacterial extracellular compounds that weaken the host’s primary or secondary defences. Invasins is the term used by medical microbiologists to describe these compounds.
  • The majority of invasins are proteins (enzymes) that operate locally to harm host cells and/or have an immediate effect of promoting the pathogen’s growth and dissemination.
  • The host harm caused by this invasive activity may contribute to the pathophysiology of an infectious disease.
  • There is no obvious distinction between the extracellular proteins produced by bacteria that facilitate their invasion and certain extracellular protein toxins (“exotoxins”) that also harm the host.
  • Invasins often function locally (in the immediate region of bacterial growth) and may not kill cells as part of their range of activity, but exotoxins are frequently cytotoxic and may act at distant sites (removed from the site of bacterial growth).
  • In addition, exotoxins are often more selective and strong in their activity compared to invasins. Nevertheless, several traditional exotoxins (e.g., diphtheria toxin, anthrax toxin) may have a role in colonisation or invasion during the initial phases of an infection, and other invasins (e.g., staphylococcal leukocidin) have a rather specific cytopathic impact.

A Survey of Bacterial Invasins

Spreading Factors

  • The name “Spreading Factors” refers to a family of bacterial enzymes that alter the physical properties of tissue matrices and intercellular gaps, hence boosting the pathogen’s dissemination.
  • Hyaluronidase. is the initial factor of propagation. It is the result of streptococci. staphylococci, and clostridia. By depolymerizing hyaluronic acid, the enzyme affects the interstitial cement (“ground material”) of connective tissue.
  • Clostridium histolyticum and Clostridium perfringens generate collagenase. It degrades collagen, the structural component of muscles, which enhances gas gangrene caused by these organisms.
  • Intestinal pathogens such as Vibrio cholerae and Shigella dysenteriae produce neuraminidase. It breaks down neuraminic acid (also known as sialic acid), an intercellular cement of the intestinal mucosa’s epithelial cells.
  • Streptococci and staphylococci generate streptokinase and staphylokinase, respectively. Kinase enzymes transform inactive plasminogen into active plasmin, which digests fibrin and inhibits blood clotting. The relative absence of fibrin in bacterial lesions that are spreading expedites the spread of pathogenic germs.

Enzymes that Cause Hemolysis and/or Leucolysis

Typically, these enzymes act on the animal cell membrane by inserting into it (creating a pore that leads to cell lysis) or by attacking phospholipids, destabilising the membrane. They may function as lecithinases or phospholipases, and if they lyse red blood cells, they are occasionally referred to as hemolysins. Staphylococcal leukocidins and streptococcal streptolysin lyse phagocytes and their granules. These last two enzymes are also considered exotoxins.

  • Phospholipases, which are produced by Clostridium perfringens (i.e. alpha toxin), remove polar head groups from phospholipids in cell membranes in order to hydrolyze them.
  • Clostridium perfringens also produces lecithinases, which degrade lecithin (phosphatidylcholine) in cell membranes.
  • Hemolysins, which are notably produced by staphylococci (e.g., alpha toxin), streptococci (e.g., streptolysin), and other clostridia, may be channel-forming proteins or phospholipases or lecithinases that lyse red blood cells and other cells (e.g., phagocytes).

Staphylococcal coagulase

  • Staphylococcus aureus produces coagulase, a cell-associated and diffusible enzyme that converts fibrinogen to fibrin, causing clotting.
  • Almost always, coagulase activity is associated with pathogenic S. aureus and practically never with nonpathogenic S. aureus. epidermidis, which has led to significant debate regarding its involvement as a virulence determinant.
  • If cell-bound coagulase coagulated fibrin on the cell surface, it might provide an antigenic mask. Or, a fibrin-encased staphylococcal lesion (such as a boil or pimple) could render the bacterial cells impervious to phagocytes, tissue bactericides, and even medications that are unable to diffuse to their intended bacterial target.

Extracellular Digestive Enzymes

  • In general, heterotrophic bacteria produce a vast array of extracellular enzymes, including as proteases, lipases, glycohydrolases, nucleases, etc., which have not been conclusively linked to invasion or disease.
  • These enzymes likely serve additional activities associated with bacterial feeding or metabolism, but they may contribute in invasion either directly or indirectly.

Toxins With Short-Range Effects Related to Invasion

  • It is believed that adenylate cyclase-active bacterial protein toxins have rapid effects on host cells that encourage bacterial invasion.
  • A component of the anthrax toxin (EF or Edema Factor) is an adenylate cyclase that operates on neighbouring cells to elevate cyclic AMP levels and alter cell permeability.
  • The agent of whooping cough, Bordetella pertussis, produces a toxin with a similar impact. The impact of these toxins on nearby macrophages and lymphocytes, which play a crucial role in containing the infection, may lead to invasion.
  • For instance, because they utilise ATP as a substrate, they may deplete phagocytes’ energy reserves, which are required for ingesting.
  • Because the rise in cAMP in afflicted cells breaks homeostasis, edoema is considered a disease.
  • The table below provides a summary of the activities of numerous bacterial proteins known for their involvement to bacterial invasion of tissues.

What is Pathogen?

  • Pathogen, in the earliest and widest biological definition, is any organism or agent that can cause disease. A pathogen is sometimes known as an infectious agent or a simple germ.
  • In the 1880s, the term pathogen came into use. Pathogen typically refers to an infectious microbe or agent, such as a virus, bacterium, protozoan, prion, viroid, or fungus.
  • In addition to humans, helminths and insects can cause or transmit disease. However, these organisms are more commonly known as parasites than pathogens.
  • Microbiology is the scientific study of microscopic organisms, especially microscopic harmful organisms, whereas parasitology is the scientific study of parasites and their host organisms.
  • There are numerous entry points for viruses to enter a host. Different episodic time frames apply to the primary pathways, but soil has the longest or most persistent potential to house a pathogen.
  • Pathogenic diseases are those that are caused by infectious pathogens in humans. Not all diseases are caused by microorganisms; for example, poisons, genetic problems, and the host’s immune system might also be responsible.

Types of pathogens


  • Algae are unicellular eukaryotes that are normally non-pathogenic, however there are pathogenic species.
  • Protothecosis is a disease observed in dogs, cats, cattle, and people that is caused by prototheca, a chlorophyll-deficient green alga.
  • The species Prototheca wickerhami is responsible for the majority of instances of the rare infection protothecosis in humans.


  • The great majority of bacteria, whose lengths range from 0.15 to 700 M, are harmless or useful to humans.
  • However, only a limited number of germs can cause infectious diseases. There are numerous ways in which pathogenic microorganisms can cause disease.
  • They can either directly harm the cells of their host, produce endotoxins that injure the cells of their host, or provoke an immunological response powerful enough to destroy the host cells.


  • Fungi are eukaryotic creatures with pathogenic potential. There are approximately 300 known fungi that are pathogenic to humans including Candida albicans, which is the most common cause of thrush, and Cryptococcus neoformans, which can cause a severe form of meningitis.
  • The average length of fungus spores is 4.7 m, but some spores may be longer.


  • Prions are transmissible misfolded proteins that can impact the improper folding of regular proteins in the brain.
  • They do not contain DNA or RNA and cannot replicate except by converting previously misfolded proteins into a misfolded form.
  • Many neurodegenerative illnesses are characterised by the presence of these improperly folded proteins, which aggregate the central nervous system and cause tissue damage by forming plaques.
  • This causes essentially “holes” in the tissue. There have been identified three modes of transmission for prions: acquired, familial, and sporadic.


  • Avoid confusion with virusoid and viral. Viroids are the tiniest known infectious microorganisms.
  • They consist just of a small, circular, single-stranded RNA molecule with no protein covering.
  • All known viroids are found in higher plants, and the vast majority cause illnesses of varying economic value to humans.


  • Viruses are typically tiny particles between 20 and 300 nanometers long that contain RNA or DNA. For viral replication, a host cell is required. Smallpox, influenza, mumps, measles, chickenpox, ebola, HIV, rubella, and COVID-19 are some of the diseases that are caused by viral pathogens.

Other parasites

  • Protozoa are unicellular eukaryotes that consume microbes and organic tissue.
  • They are considered “one-celled animals” because they exhibit animal-like characteristics like as movement, predation, and the absence of a cell wall.
  • Malaria, amoebiasis, giardiasis, toxoplasmosis, cryptosporidiosis, trichomoniasis, Chagas disease, leishmaniasis, African trypanosomiasis (sleeping sickness), Acanthamoeba keratitis, and primary amoebic meningoencephalitis are some of the diseases caused by protozoan infections (naegleriasis).

What is Pathogenicity?

  • Pathogenicity refers to an organism’s capacity to produce disease (ie, harm the host).
  • This ability represents a genetic component of the pathogen, whereas the overt damage caused to the host is a consequence of the host-pathogen interactions. Commensals and opportunistic pathogens lack this innate disease-causing capability.
  • Pathogenicity refers to a pathogenic agent’s capacity to produce disease. Infectious bacteria, viruses, prions, fungus, viroids, and parasites are examples of pathogenic agents.
  • Their potential to cause disease is tied to the qualities they acquired while attempting to survive in their host. The host may be a particular animal, plant, fungus, or microorganism.
  • Virulence, which relates to the degree of pathogenicity of a given organism, is a related term to pathogenicity.
  • Certain disease-causing microorganisms, for instance, are capable of generating and releasing toxins, infiltrating tissues, competing for nutrition, and suppressing their host’s immune system.
  • Pathogenic microbial species are those that cause diseases such as smallpox, measles, rubella, Ebola, etc. For instance, the human immunodeficiency virus (HIV) that causes HIV infection and acquired immunodeficiency syndrome (AIDS) is capable of infecting immune system cells, e.g. T cells. CD4+ T cells, macrophages, and dendritic cells.
  • Not only are they capable of evading the immune response and process of such cells, but they are also capable of destroying them. They are also capable of entering, multiplying, and killing these cells.

What is Virulence?

  • In biology, virulence is defined as the pathogenic organism’s ability to cause disease. The phrase derives from the Latin vrulentus, which means “poisonous” or “toxic.” A related term, virulent, is a derivative term used to indicate that a pathogen is highly toxic. Similar terms: virulence
  • Pathogenicity is related to virulence in the sense that the notion of virulence is associated with the manifestation of a disease.
  • However, pathogenicity is specifically defined as a pathogen’s ability to cause disease. A pathogen is an organism that produces disease and causes harm to its host.
  • In order to remain within the host, the capacity to cause disease is related with the organism’s innate traits. In contrast, virulence refers to the pathogenicity of a certain organism.
  • A virulent pathogen is an organism that causes much more damage to its host than non-pathogenic organisms.
  • Pathogenic organisms exhibit varying degrees of virulence. A strain of bacteria, for instance, may be more virulent than other strains of the same species.
  • Pathogenicity variables are frequently connected with the virulence of a pathogen. A virulence factor is a factor that allows an organism to infiltrate its host and cause disease. It also determines the extent of host harm.
  • The nature of these factors may be secretory, membrane-associated, or cytoplasmic.
  • The capacity of microorganisms to multiply within their host cells is an example of a virulence factor. In microbiology, these variables are regarded as crucial to epidemiology, especially when tracing a novel pathogenic strain.
  • This is due to the fact that the strain is frequently more virulent and, as a result, more damaging, even lethal, to its host. Among the virulence factors investigated by scientists are the route of entry into the host, the pathobiological machinery utilised, and the consequences on the host’s immune system.
  • Viral virulence factors, for example, are mostly proteins that the infecting virus induces the host’s protein machinery to create. Similarly, bacterial virulence factors are proteins encoded by their own genes or by plasmids acquired by horizontal gene transfer.
  • When the immune cells are so activated by the presence of these virulence factors that they tend to damage the host cells in an effort to combat the infection, the host’s immunological response may exacerbate the damage.
  • Therefore, these virulence factors are one of the primary objectives of medical research that aims to develop new therapies and vaccines.
  • Human immunodeficiency virus (HIV) is a pathogenic virus. It is the agent responsible for AIDS. It is virulent because it employs ways to evade the immune cells of the host. For example, it infects T-helper cells, an immunological cell type. Thus, the host’s immunological response is already diminished and impaired.
  • Another example is the rabies-causing lyssavirus. It penetrates and hijacks muscle cells, and then travels across neuromuscular junctions to the nervous system. Thus, it is characterised as neurovirulent, or capable of causing sickness in the neurological system.
  • As for bacteria, Mycobacterium tuberculosis (the causative agent of tuberculosis) and Bacillus anthracis are two examples of human pathogens (causative agent of anthrax).

Methods by which bacteria cause disease


  • Many bacteria must first adhere to the surfaces of host cells. Numerous bacterial and host substances that contribute to bacterial adherence to host cells have been found.
  • Frequently, the bacterial receptors on the surface of the host cell are important proteins for other tasks.
  • Due to the existence of mucus lining and antimicrobial compounds around particular host cells, direct contact-adhesion is difficult for certain infections.


  • Some virulent bacteria create proteins that allow them to invade the host’s tissues.
  • By manufacturing the enzyme urease, Helicobacter pylori is able to thrive in the acidic environment of the human stomach.
  • This bacterium’s colonisation of the stomach lining can result in gastric ulcers and cancer.
  • The virulence of certain Helicobacter pylori strains correlates with the amount of urease they produce.


  • Some aggressive bacteria release proteins that damage the membranes of host cells or encourage their own endocytosis or macropinocytosis into host cells.
  • These virulence factors enable bacteria to infiltrate host cells and facilitate entrance into the body via epithelial tissue layers at the body’s surface.

Immune response inhibitors

  • Numerous bacteria produce virulence factors that suppress the immunological defences of the host. An example of a common bacterial tactic is the production of host-antibody-binding proteins.
  • Streptococcus pneumoniae’s polysaccharide capsule hinders phagocytosis of the bacteria by host immune cells.


  • Numerous bacterial virulence factors are proteins that toxify host cells and induce tissue damage. For instance, bacteria produce numerous food poisoning toxins that can infect human foods.
  • Some of these can survive cooking in “spoiled” food and cause illness when the infected food is consumed.
  • Other bacterial toxins are chemically changed and rendered inactive by cooking heat.

What is toxigenicity?

  • The capacity of a bacterium to create a toxin that contributes to disease development.

What are Carriers?

  • A carrier is an individual with an undetectable infection who is able to transfer the pathogen to others.
  • Carriers frequently pose a threat to public health because they can infect others.
  • Numerous outbreaks have been attributed to carriers. Mary Mallon was responsible for a tiny outbreak of typhoid everywhere she worked as a maid.
  • Early-infected children become Hepatitis B carriers and have a greater chance of developing hepatocellular cancer.
  • In hospitals, if a member of the health care staff is a carrier, that person may become a source of infection for numerous patients. Those who are nasal carriers of S. aureus tend to experience recurrent infections.

Classification of Carriers

  1. Healthy carriers: These are the individuals who contain the bacterium but have never contracted a sickness from it.
  2. Convalescent carriers: Convalescent carriers are individuals who have been infected by the pathogen and are currently recuperating. Even if they no longer exhibit symptoms, they continue to release the bacteria into the environment.
  3. Temporary carriers: Temporary carriers have a duration of less than six months.
  4. Chronic carriers: the duration of the carrier condition exceeds six months and can endure for years.
  5. Contact carrier: A person can become a contact carrier if he receives the pathogen from the patient through direct touch.
  6. Paradoxical carrier: A person is a paradoxical carrier if he receives the bacterium from another paradoxical carrier.

Sites of carriage

  1. Nasal: Staphylococcus aureus
  2. Nasopharyngeal: meningococcus, Bordetella
  3. Urinary: Salmonella
  4. Fecal: Salmonella
  5. Serum: hepatitis B
  6. Hands: Klebsiella, Pseudomonas, Staphylococcus

What are opportunistic infections?

  • Opportunistic infections (OIs) are illnesses that are more prevalent and severe in HIV-positive individuals. This is because their immune systems are compromised.
  • AIDS-related opportunistic infections (OIs) are less prevalent among HIV-positive individuals due to the effectiveness of HIV treatment.
  • However, some HIV-positive individuals continue to acquire OIs because
    • They may be unaware of their HIV status.
    • They may not be taking HIV medication, or
    • Their HIV treatment may not be effective.

Prevention of opportunistic infections

Taking HIV medication is the most effective strategy to prevent OIs. HIV medications can maintain a robust immune system. If you develop an OI, discuss treatment options with your healthcare provider. Additionally, there are measures you can take to prevent OIs:

  • Consult your healthcare professional about preventative medications and immunizations for certain OIs.
  • Prevent the transmission of more sexually transmitted illnesses.
  • Sharing needles, syringes, or other drug injecting equipment is prohibited (for example, cookers).
  • Limit your exposure to potentially fatal pathogens. This includes tuberculosis and bacteria found in animal faeces, saliva, or on their skin.
  • Certain foods, such as undercooked eggs, raw milk and cheeses, unpasteurized fruit juices, and raw seed sprouts, should not be consumed.
  • Do not consume untreated water, such as that from lakes or rivers. Avoid drinking tap water when travelling abroad. Use bottled or filtered water.
  • Discuss with your doctor the factors that could expose you to OIs at work, at home, and on vacation.

Common Opportunistic Infections


  • Candidiasis is caused by an infection with the Candida fungus.
  • Candidiasis can affect the entire body, including the skin, nails, and mucous membranes.
  • Candida infections are common in HIV-positive individuals, particularly in the mouth and vagina.
  • Candidiasis is only regarded an OI when it causes serious or persistent infections in the mouth or vagina, or when it develops in the oesophagus, lower respiratory tract, trachea, bronchi, or deeper lung tissue.

Invasive cervical cancer

  • Cervical cancer begins in the cervix (the lowest portion of the uterus at the top of the vagina) and spreads to other areas of the body.
  • Cervical cancer can be prevented by having frequent cervix checks performed by a physician.


  • This condition is brought on by the fungus Coccidioides.
  • It is also known as San Joaquin Valley fever, valley fever, and desert fever.
  • People can contract it by inhaling fungus spores.
  • Particularly prevalent in hot, arid parts of the southwestern United States, Central America, and South America.


  • This condition is caused by an infection with Cryptococcus neoformans.
  • Typically, the fungus enters the body via the lungs and can cause pneumonia.
  • Cryptococcosis typically affects the lungs or the central nervous system (brain and spinal cord), although it can affect other sections of the body as well.

Cryptosporidiosis (Crypto)

  • Crypto is a diarrheal illness caused by the microorganism Cryptosporidium.
  • Abdominal pains and severe, persistent, watery diarrhoea are among the symptoms.


  • Formerly referred to as isosporiasis
  • The parasite Cystoisospora belli causes this infection (formerly known as Isospora belli).
  • Cystoisosporiasis can be transmitted to the body via contaminated food or water.
  • Included in the list of symptoms are diarrhoea, fever, headache, abdominal pain, vomiting, and weight loss.

Cytomegalovirus (CMV)

  • CMV can infect various organs and cause pneumonia, gastroenteritis (particularly stomach pain due to infection of the colon), encephalitis (infection) of the brain, and sight-threatening retinitis (infection of the retina at the back of eye).
  • Vision impairments associated with CMV retinitis intensify over time. CMV retinitis is a medical emergency because, if left untreated, it can lead to blindness.

Encephalopathy, HIV-related

  • This brain condition can emerge either as a result of acute HIV infection or chronic HIV infection.
  • Its specific aetiology is unknown, but it is believed to be associated to HIV infection and subsequent brain inflammation.

Herpes simplex virus (HSV)

  • HSV is a common virus that produces little issues for the majority of people.
  • HSV is typically transmitted sexually or from mother to child at birth.
  • In the majority of individuals with healthy immune systems, HSV is often dormant (inactive).
  • Stress, trauma, other infections, or immune suppression (such as HIV) might reactivate the dormant virus and cause symptoms to resurface.
  • HSV can produce severe cold sores (also referred to as fever blisters) in or around the mouth, as well as painful ulcers in or around the genitalia or anus.
  • HSV can also cause infection of the bronchus (breathing tube), pneumonia (infection of the lungs), and esophagitis in individuals with severely compromised immune systems (infection of the esophagus, or swallowing tube).


  • Histoplasma is the fungus that causes histoplasmosis.
  • Histoplasma typically manifests in the lungs and causes flu-like or pneumonia-like symptoms.
  • People with significantly compromised immune systems are susceptible to developing progressive disseminated histoplasmosis. This kind of histoplasmosis can persist for an extended period of time and spread to other areas of the body.

Kaposi’s sarcoma (KS)

  • The virus Kaposi’s sarcoma herpesvirus (KSHV) or human herpesvirus 8 causes KS (HHV-8).
  • KS causes abnormal growth of tiny blood vessels and can occur anywhere in the body.
  • KS manifests as hard, pink or purple spots that may be elevated or flat on the skin.
  • When KS affects internal organs, such as the lungs, lymph nodes, or intestines, it can be fatal.


  • Lymphoma is cancer of lymph nodes and other lymphoid tissues.
  • There are numerous types of lymphoma. Some lymphomas, including non-Hodgkin lymphoma and Hodgkin lymphoma, are linked to HIV.

Tuberculosis (TB)

  • Mycobacterium tuberculosis is the bacteria that causes tuberculosis.
  • TB can be transmitted through the air when an infected person coughs, sneezes, or speaks. Inhaling the bacteria can result in a lung infection.
  • TB in the lungs is characterised by cough, fatigue, weight loss, fever, and night sweats.

Mycobacterium avium complex (MAC)

  • Mycobacterium avium, Mycobacterium intracellulare, or Mycobacterium kansasii infections are the cause of MAC.
  • These microorganisms inhabit our surroundings, including soil and dust.
  • Infections caused by these bacteria can be fatal for those with compromised immune systems, as they travel throughout the body and endanger organ systems.

Pneumocystis pneumonia (PCP)

  • PCP is an infection of the lungs brought on by the fungus Pneumocystis jirovecii.
  • PCP affects individuals with compromised immune systems.
  • The initial symptoms of an infection are trouble breathing, a high fever, and a dry cough.


  • Pneumonia is an infection that can affect either lung.
  • Numerous microorganisms, such as bacteria, viruses, and fungus, can cause pneumonia.
  • A cough (with mucus), fever, chills, and difficulty breathing are among the symptoms.
  • Infection with Streptococcus pneumoniae, commonly known as Pneumococcus, is one of the most prevalent and life-threatening causes of pneumonia in persons with highly compromised immune systems due to HIV. People with HIV should receive a vaccination to prevent Streptococcus pneumoniae infection.

Progressive multifocal leukoencephalopathy

  • This uncommon condition of the brain and spinal cord is caused by the JC (John Cunningham) virus.
  • Almost primarily affects individuals whose immune systems have been significantly compromised by HIV.
  • Symptoms may include loss of muscle control, paralysis, blindness, speech difficulties, and a mental state alteration.
  • This disease frequently advances swiftly and is potentially lethal.

Salmonella septicemia

  • Salmonella are bacteria that primarily enter the body through consuming infected food or water or by drinking contaminated water.
  • Salmonella infection (also known as salmonellosis) can affect anyone and typically results in nausea, vomiting, and diarrhoea.
  • Salmonella septicemia is a severe form of infection in which the germs circulate throughout the entire body and are unable to be controlled by the immune system.


  • This disease is brought on by the parasite Toxoplasma gondii.
  • The parasite is transmitted through the excrement of warm-blooded animals, such as cats, rats, and birds (stool).
  • People can contract it through inhaling contaminated dust or consuming infected food.
  • Toxoplasma can also be found in commercially produced meats, particularly red meats and pig, but seldom in poultry.
  • Lungs, retina of the eye, heart, pancreas, liver, colon, testicles, and brain are susceptible to infection.
  • Even though cats can spread toxoplasmosis, litter boxes can be changed securely by wearing gloves and thoroughly cleaning hands with soap and water afterwards.
  • All raw red meats not frozen for at least 24 hours must be cooked to an internal temperature of at least 150 degrees Fahrenheit.

Wasting syndrome due to HIV

  • Wasting is defined as the involuntary loss of more than 10% of one’s body weight while having experienced diarrhea or weakness and fever for more than 30 days.
  • Wasting refers to the loss of muscle mass, albeit a portion of the weight loss may also be attributable to fat loss.

What is Nosocomial Infections?

  • A nosocomial infection is an infection acquired during an unrelated hospitalisation. It is also known as a healthcare-associated infection or hospital-acquired infection.
  • Patients and medical staff bring pathogens into hospitals and spread them to one another.
  • Sometimes, people carry these viruses without feeling ill, which means they unknowingly transmit them to others.
  • If you are already hospitalised for surgery or another illness, your immune system may be too compromised to fight off these pathogens.
  • Nosocomial infections occur when these microorganisms cause illness within 48 hours after entering a hospital.
  • One is available at any healthcare establishment. If left untreated, these infections might lead to more significant health problems.

Types of Nosocomial Infections

There are multiple prevalent forms of nosocomial infections, including:

  • Bacterial infections: Infections by bacteria Bacteria are microscopic organisms that are invisible to the naked eye. Many are harmless, but a few can cause severe sickness. The most common cause of nosocomial infections is bacteria. Common bacteria include E. coli and Salmonella. coli and staph bacteria.
  • Fungal infections: Candida infections Fungi consist of living organisms such as mushrooms, mould, and yeast. Certain fungus can cause infectious diseases. Candida (thrush) and Aspergillus are the fungus that most frequently cause nosocomial infections.
  • Viral diseases: Viruses are small microorganisms that replicate their host’s genetic code to spread throughout the body. They deceive your body into producing copies of them, just as it does with other cells. Viruses can result in serious illness.

Common viral nosocomial infections include influenza (flu) and respiratory synctial virus.

Symptoms of Nosocomial Infections

Symptoms of nosocomial infections vary based on their nature. Among the most prevalent symptoms are:

  • Fever.
  • Sensation of burning while urinating.
  • Cough.
  • Extreme exhaustion or weakness.
  • Redness and pain of the skin surrounding a surgical or needle wound.
  • Sweating.
  • Muscle discomfort.
  • sickness and vomiting.

Causes of Nosocomial Infections

There are numerous causes of nosocomial infections in a hospital setting.

  • Antibiotics: A physician may prescribe antibiotics to prevent or treat an illness while you are hospitalised. Many types of bacteria in the human body are beneficial and can eliminate dangerous germs. Antibiotics increase the likelihood of nosocomial infection because they eliminate both good and pathogenic microorganisms in the body.
  • Urinary catheterizatio: These are tubes placed into the bladder via the urethra. It can be useful during operations or other treatments when you are unable to get up frequently to use the restroom. Leaving a catheter in place for too long can result in a bacterial urinary tract infection.
  • Breathing apparatuses: Ventilators are equipment that assist with respiration by forcing air into and out of the lungs. A ventilator can harbour bacteria that can enter the body. This may result in pneumonia.
  • Central lines: A central line is a tube that is inserted into the neck, chest, arm, or groyne in order to administer medication directly into the bloodstream. Through the tube, serious bloodstream infections can be transmitted. These microorganisms can thrive on medical gloves, the skin where the catheter is implanted, and the end of the catheter.
  • Not thoroughly cleaning before surger: Surgical procedures entail cutting the skin. Before surgery, germs can enter the body if your skin, hair, or surgical instruments are not thoroughly cleaned.

Treatment for Nosocomial Infections

  • Antibiotics: This is a standard treatment for nosocomial infections. Medical tests assist doctors in identifying the exact germs responsible for your ailment. Then, your doctor can give antibiotics that target only the bad germs and leave the healthy bacteria alone.
  • Rest: In order to recover from an infection, you will likely require rest. Physical rest enables the immune system to combat disease with maximum vigour.
  • Fluids: Water is necessary for the body to combat an infection. Water helps keep your body cool during a fever and hydrates your airways to prevent coughing. Your physician may recommend that you drink plenty of water. You may also need to receive fluids via intravenous infusion.

Most common type of nosocomial infection

Although other bacteria, viruses, and fungi can cause nosocomial infections, Staphylococcus aureus is the most prevalent. Other typical causes include Escherichia coli, Enterococci, and Candida, all of which are normally present on the skin and mucous membranes. Antibiotic-resistant bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA), are particularly harmful and challenging to treat.

Urinary catheters

  • A urinary catheter is a tube put into the bladder that collects urine in a closed system. Patients who have trouble regulating or emptying their bladder may benefit from urinary catheters. Due to the inability of anaesthetized patients to regulate their bladders, urine catheters are frequently inserted during surgical procedures to empty the bladder.
  • Urine tract infections are the most frequent nosocomial infections, and they can be caused by pathogens transmitted through the perineum or a contaminated urinary catheter.
  • Urinary tract infections are characterised by painful urination, flank pain, and fever.

Surgical procedures

  • Surgical site infections are the second most common type of postoperative infection.
  • The incidence of surgical site nosocomial infections can be affected by the duration of surgery, surgical technique, and sterility of the operating room.
  • Surgical site infections are typically associated with the skin, organs, or implanted materials, and are caused by pathogens already present on the skin or organisms shed by members of the operating room staff.
  • Redness, pain, and drainage from surgical sites may be symptoms.

Central venous catheters

  • A central venous catheter (also known as a central line) is a tube that is inserted into a major vein in the neck, arm, chest, or groyne and can remain in place permanently. Intravenous therapy such as total parenteral nutrition (TPN), which delivers nutrients and fluids to patients, can be administered using central venous catheters.
  • Infections of the bloodstream can be caused by bacteria that penetrate the skin during the insertion of central line hubs. This type of nosocomial infection is the third most prevalent and has the greatest fatality rate.
  • At insertion sites, an infection may manifest as redness, pain, and drainage.

Mechanical ventilation

  • Inhaling infected oropharyngeal flora during mechanical ventilation causes ventilator-associated pneumonia (machine-assisted breathing).
  • It is the third most prevalent nosocomial infection, along with central-line bloodstream infections.
  • Early-onset nosocomial pneumonia is typically caused by community-acquired bacteria such as Staphylococcus aureus, Streptococcus pneumoniae, and Haemophilus influenzae and occurs during the first four days after admission.
  • Multidrug-resistant bacteria like MRSA, Pseudomonas aeruginosa, Klebsiella, and Acinetobacter frequently cause late-onset pneumonia.
  • Symptoms include fever, an increase in mucus production, an increase in the number of white blood cells, and abnormal chest X-ray results.

Preventing Nosocomial Infections

Doctors and other medical personnel can prevent the transmission of nosocomial illnesses by:

  • Completely sterilising skin and equipment.
  • Hand washing regularly
  • Utilizing protective gear such as face masks and gloves.
  • Changing catheters frequently and removing them as quickly as feasible.
  • Hair removal around a surgical site.
  • Only prescribing antibiotics when necessary.

How Are Diseases Transmitted?

  • An infectious disease agent can be transmitted in two ways: as horizontal disease agent transmission from one individual to another in the same generation (peers in the same age group) by either direct contact (licking, touching, biting), or indirect contact through air – cough or sneeze (vectors or fomites that allow the transmission of the agent causing the disease without physical contact) or by vertical disease transmission, passing the agent causing the disease from parent to offspring, such as in prenatal or perinatal transmission.
  • Infectivity refers to the capacity of an organism to enter, survive, and reproduce within a host, whereas infectiousness refers to the relative ease with which a disease agent can be transmitted to additional hosts.
  • Pathogens can be transmitted through direct contact, contaminated food, body fluids, or items, airborne inhalation, or vector organisms.
  • Given contact between an infected host and a non-infected host, transmissibility is the possibility of an infection.
  • Community transmission indicates that the source of infection for the spread of a disease is unknown or that there is no contact between patients and other individuals.
  • It refers to the difficulty in understanding the epidemiological link beyond confirmed cases.
  • Local transmission indicates that the infection’s source has been identified inside the reporting location (such as within a country, region or city).

Direct contact Transmission

Infectious infections are frequently transmitted through personal contact. Included in direct interaction are the following.

1. Person-to-person contact

  • Infectious infections are typically transmitted through direct contact between individuals. When a person with an infectious condition touches or exchanges body fluids with another person, transmission occurs.
  • This may occur before the individual is aware of their sickness. This method can transmit sexually transmitted diseases (STDs) and gastrointestinal illnesses.
  • Additionally, pregnant women can transfer infectious illnesses to their foetuses via the placenta. Some STDs, including gonorrhoea, can be transmitted from the mother to the kid during childbirth.

2. Droplet spread

  • The spray of droplets produced by coughing and sneezing has the potential to spread an infectious disease. Even the droplets produced while you talk can infect another individual.
  • Due to the fact that droplets fall to the ground within a few feet, close proximity is required for this form of transmission.

Indirect contact

Infectious diseases can also be transmitted indirectly via the air and other means. For instance:

1. Airborne transmission

  • Some infectious pathogens can fly great distances and remain airborne for extended periods of time.
  • You can catch a disease such as measles by entering a room after an infected person has left.

2. Contaminated objects

  • Some species can survive briefly on objects. You may be exposed to infection if you touch an object, such as a doorknob, shortly after a person with an infectious sickness.
  • When you touch your mouth, nose, or eyes without first thoroughly washing your hands, transmission happens.
  • Viruses and bacteria can also be transmitted through blood products and medical supplies.

3. Food and drinking water

  • Food and water harbouring the virus or bacterium can transmit infectious illnesses. coli is frequently transferred through carelessly handled food or raw meat.
  • Incorrectly canned foods can foster the growth of Clostridium botulinum, which can cause botulism.

4. Animal-to-person contact

  • There are infectious diseases that can be transmitted from animals to humans. This can occur if an infected animal bites or scratches you, or if you handle animal excrement.
  • The parasite Toxoplasma gondii can be discovered in cat faeces.
  • Pregnant women and anyone with impaired immune systems should avoid changing kitty litter or take extra precautions (disposable gloves and thorough handwashing) when doing so.

5. Animal reservoirs

Sometimes, animal-to-human disease transmission can spread to humans. Zoonosis is the transmission of illnesses from animals to humans. Zoonotic illnesses consist of:

  • anthrax (from sheep) (from sheep)
  • rabies (from rodents and other mammals) (from rodents and other mammals)
  • West Nile virus (from birds)
  • plague (from rodents) (from rodents)

6. Insect bites (vector-borne disease)

  • Certain zoonotic infectious agents are transmitted by insects, particularly bloodsucking insects. Included in this category are mosquitoes, fleas, and ticks.
  • When insects feed on infected hosts, such as birds, animals, and humans, they get infected. When the bug bites a new host, the sickness is subsequently transferred.
  • This is how malaria, West Nile virus, and Lyme disease are transmitted.

7. Environmental reservoirs

  • People can also be exposed to infectious organisms in the soil, water, and vegetation.
  • For example, hookworm is transmitted through infested soil. Legionnaires’ disease is an example of a disease that can be transmitted by cooling tower and evaporative condenser water.

Beneficial microorganisms

Important to the biology of beneficial microbial symbionts, such as coral-associated dinoflagellates and human microbiota, is the route of transmission. Organisms can create symbioses with germs from their parents, the environment, unrelated persons, or both.

Vertical transmission

  • Vertical transmission refers to symbiont acquisition from parents (usually mothers). Vertical transmission might be intracellular (e.g. transovarial), or extracellular (for example through post-embryonic contact between parents and offspring).
  • Both intracellular and extracellular vertical transmission are examples of non-genetic inheritance or parental impact.
  • It has been suggested that symbionts are transmitted vertically in the majority of species.
  • The nutritional symbiont Buchnera in aphids (transovarially transferred intracellular symbiont) and several components of the human microbiome are canonical examples of vertically transmitted symbionts (transmitted during passage of infants through the birth canal and also through breastfeeding).

Horizontal transmission

  • Some advantageous symbionts are acquired horizontally, from the environment or from unrelated people. This necessitates that the host and symbiont have a means of recognising one another or their respective products or services.
  • However, some primary nutritional symbionts are also horizontally (environmentally) acquired. Typically, horizontally acquired symbionts are relevant to secondary rather than primary metabolism, such as defence against pathogens, but some primary nutritional symbionts are also horizontally (environmentally) acquired.
  • Bioluminescent bacteria linked with bobtail squid and nitrogen-fixing bacteria in plants are two other examples of horizontally transmitted beneficial symbionts.

Mixed-mode transmission

  • Many microbial symbionts, including human microbiota, are transmissible in both vertical and horizontal directions.
  • Mixed-mode transmission can provide symbionts with the “best of both worlds” – they can vertically infect host offspring when host density is low and horizontally infect a variety of different hosts when a large number of additional hosts are available.
  • Mixed-mode transmission makes it more difficult to anticipate the relationship’s result (degree of harm or benefit) because the evolutionary success of the symbiont is sometimes, but not always, connected to that of the host.


  • Southwood, D., & Ranganathan, S. (2018). Host-Pathogen Interactions. Reference Module in Life Sciences. doi:10.1016/b978-0-12-809633-8.20088-5
  • Abreu, R., Giri, P., & Quinn, F. (2020). Host-Pathogen Interaction as a Novel Target for Host-Directed Therapies in Tuberculosis. Frontiers in Immunology, 11. doi:10.3389/fimmu.2020.01553
  • Casadevall A, Pirofski LA. Host-pathogen interactions: basic concepts of microbial commensalism, colonization, infection, and disease. Infect Immun. 2000 Dec;68(12):6511-8. doi: 10.1128/IAI.68.12.6511-6518.2000. PMID: 11083759; PMCID: PMC97744.
  • Mallon, C. A., Elsas, J. D. van, & Salles, J. F. (2015). Microbial Invasions: The Process, Patterns, and Mechanisms. Trends in Microbiology, 23(11), 719–729. doi:10.1016/j.tim.2015.07.013

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