∞ generated and posted on 2022.01.23 ∞

Life form that is neither a cell nor consists of cells and which minimally consists of a combination of a nucleic-acid genome and protein coat.

Viruses are obligate intracellular parasites of cellular organisms, but do not themselves consist of cells, and must possess some form of extracellular state, and a protein capsid, to – though with a few exceptions – to be considered a virus.

Viruses infect cells, that is, they deliver their nucleic acid genome into cells where the genome then expresses virus genes, turning the infected cell into a virus-making "factory".

Figure legend: Bacteriophage T4 as a computer-generated image, generated by the master of phage graphics himself, Steven McQuinn.

Distinguishing virus characteristics include:

Though many viruses cause diseases, and do so across the diversity of cellular organisms, in fact most viruses are severely limited in terms of what kinds of cellular organisms they can infect (re: host range). Note that capsid, at least in part, is another name for virus "coat".

Not all viruses cause disease even in all organisms they infect or under all circumstances even in those organisms they otherwise do have a potential to cause disease in. Nevertheless, see Virus-caused diseases.

Norrby (2008) (p. 1110) provides this quote from André Lwoff from 1957 defining viruses as well as some discussion after:

'…infectious, potentially pathogenic, nucleoproteinic entities possessing only one type of nucleic acid, which are reproduced from their genetic material, are unable to grow and to undergo binary fission, and are devoid of a Lipmann system.' The last part of the statement notes that viruses have no energy converting systems and generally very limited metabolic functions. However, they do have the qualities of life associated with replication and capacity to show genetic variation.

The following video does fly off the "deep end" on a number of occasions, but nonetheless has pretty pictures and more or less gets things right: