A microorganism smaller than a bacteria, which cannot grow or reproduce apart from a living cell. A virus invades living cells and uses their chemical machinery to keep itself alive and to replicate itself. It may reproduce with fidelity or with errors (mutations)-this ability to mutate is responsible for the ability of some viruses to change slightly in each infected person, making treatment more difficult.
Viruses cause many common human infections, and are also responsible for a bevy of rare [disease]]s. Examples of viral illnesses range from the common cold, which is usually caused by one of the rhinoviruses, to acquired immunodeficiency syndrome (AIDS), which is caused by the human immunodeficiency virus (HIV).
Viruses may contain either DNA or RNA as their genetic material. Herpes simplex virus and the hepatitis- B virus are DNA viruses. RNA viruses have an enzyme called reverse transcriptase that permits the usual sequence of DNA-to-RNA to be reversed so the virus can make a DNA version of itself. RNA viruses include HIV and the hepatitis C virus.
Researchers have grouped viruses together into several major families, based on their shape, behavior, and other characteristics. These include the herpesviruses, adenoviruses, papovaviruses (papilloma viruses), hepadnaviruses, poxviruses, and parvoviruses among the DNA viruses. On the RNA virus side, major families include the picornaviruses (including the rhinoviruses), calciviruses, paramyxoviruses, orthomyxoviruses, rhabdoviruses, filoviruses, bornaviruses, and retroviruses. There are dozens of smaller virus families within these major classifications. Many viruses are host-specific, causing disease in humans or specific animals only.
The Dutch microbiologist Martinus Beijerinck (1851-1931) was the first person to use the term “virus” for the invisible disease-causing material that he showed to be self-replicating.