Which microbes are not included in the three domain system?

The three-domain system classifies all known cellular life into three domains: Archaea, Bacteria, and Eukarya. Viruses, however, are not included in this system because they do not exhibit the fundamental characteristics of living organisms. They lack cellular structure, do not possess ribosomal RNA (rRNA), and cannot carry out metabolic processes independently. Viruses require a host cell to replicate and reproduce, placing them outside the scope of the three-domain system.