A virus is a small infectious agent that reproduces inside the cells of living hosts. Once infected, the host cell is involuntary against rapidly produced thousands of identical duplicates of the original virus. Unlike most living things, viruses do not have cells that split or divide but instead new viruses assemble in the infected host cell. Nevertheless, unlike simpler infectious agents like prions, they contain genes, which let them mutate and evolve. Over 4,800 species of viruses have been discovered.

Viruses spread in many more ways. Just as many are very specific as to which host species or tissue they attack, each species of virus depend on a specific way for propagation. Viral infections can be a source of disease in humans, animals and plants. In humans and animals, they are usually reduced by the immune system, deliberating lifetime immunity to the host for that virus. Antibiotics have no accounting effect, but antiviral drugs can treat life-threatening infections. Vaccines that produce lifelong immunity can stop some infections. Viruses such as influenza are spread over the air by droplets of moisture when people cough or sneeze.