A virus is a biological agent that reproduces inside the cells of living hosts. When infected by a virus, a host cell is forced to produce many thousands of identical copies of the original virus, at an extraordinary rate. Unlike most living things, viruses do not have cells that divide; new viruses are assembled in the infected host cell. Over 2,000 species of viruses have been discovered.

A virus consists of two or three parts: all viruses have genes made from either DNA or RNA, long molecules that carry the genetic information; all have a protein coat that protects these genes; and some have an envelope of fat that surrounds them when they are not within a cell. Viruses vary in shape from the simple helical and icosahedral to more complex structures. Viruses are about 100 times smaller than bacteria, and it would take 30,000 to 750,000 of them, side by side, to stretch to 1 centimetre (0.39 in).