Viruses

Acellular infectious particles: genetic material, DNA or RNA, wrapped in a protein coat, unable to replicate without hijacking a host cell's own machinery. Classified into species by the International Committee on Taxonomy of Viruses (ICTV) since 1991; covers every ICTV-recognized virus regardless of host, from bacteriophages to human pathogens like influenza A virus and HIV-1. Excludes viroids and prions, which share the acellular reasoning but lack a virus's own protein coat and, in a prion's case, any nucleic acid at all; excludes every cellular domain of life already covered by the other sixteen groups above.