Virus


 * 1) formerly, the specific agent of an infectious disease
 * 2) specifically, a term for a group of infectious agents, 15 to several hundred nanometers in size, which with few exceptions are capable of passing through fine filters that retain most bacteria, are usually not visible through the light microscope, lack independent metabolism, are incapable of growth or reproduction apart from living cells, with remnants of a prokaryotic genetic apparatus, a genome of nucleic acids (either DNA or RNA) and a protein shell or capsid for its protection in a virion particle; classification by physio-chemical characteristics of virions, by mode of transmission, host range, symptomatology, and other factors See retrovirus
 * 3) relating to or caused by a virus, as a virus disease
 * 4) (obsolete usage) before the era of bacteriology, any agent causing disease, including a chemical substance such as an enzyme similar to snake venom; synonymous at that time with poison.