Definition of Virus

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Virus: 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 diseases. 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.