A virus (from the Latin noun virus, meaning toxin or poison) is a sub-microscopic particle (ranging in size from 20–300 nm) that can infect the cells of a biological organism. Viruses can replicate themselves only by infecting a host cell. They therefore cannot reproduce on their own. At the most basic level, viruses consist of genetic material contained within a protective protein coat called a capsid. They infect a wide variety of organisms: both eukaryotes (animals, plants, protists, and fungi) and prokaryotes (bacteria and archaea). A virus that infects bacteria is known as a bacteriophage, often shortened to phage. The study of viruses is known as virology and people who study viruses are known as virologists. Viruses cause several serious human diseases, such as AIDS, influenza and rabies. Therapy is difficult for viral diseases as antibiotics have no effect on viruses and few antiviral drugs are known. The best way to prevent viral diseases is with a vaccine, which produces immunity.
It has been argued extensively whether viruses are living organisms. Most virologists consider them non-living, as they do not meet all the criteria of the generally accepted definition of life. For example, unlike living organisms as defined, viruses do not respond to changes in the environment.