virus

A virus (from the Latin virus meaning "toxin" or "poison"), is a sub-microscopic infectious agent that is unable to grow or reproduce outside a host cell. Each viral particle, or virion, consists of genetic material, DNA or RNA, within a protective protein coat called a capsid. The capsid shape varies from simple helical and icosahedral forms, to more complex structures with tails or an envelope. Viruses infect cellular life forms and are grouped into animal, plant and bacterial types, according to the type of host infected.

Biologists debate whether or not viruses are living organisms. Some consider them non-living as they do not meet the criteria of the definition of life. For example, unlike most organisms, viruses do not have cells. However, viruses have genes and evolve by natural selection. Others have described them as organisms at the edge of life. Viral infections in human and animal hosts usually result in an immune response and disease. Often, a virus is completely eliminated by the immune system. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. Vaccines that produce lifelong immunity can prevent viral infections.