Viruses are the smallest infectious agents and contain only one kind of nucleic acid (RNA or DNA) as their genome. The nucleic acid is encased in a protein shell, which may be surrounded by a lipid-containing membrane (envelop). Viruses are inert in the extracellular environment; they replicate only in living cells, being parasites at the genetic level. The term virus, which come from the latin word for venom (poison). Because the viruses pass through bacterial filters, therefore the viruses were known as (filterable agents). Medical Virology :- The science that deal with the study of the medically viruses which infect human. Virus is a broad general term for any aspect of the infectious agent and includes:- • The infectious or inactivated virus particle. • Viral nucleic acid and protein in the infected cell.
