A group of submicroscopic pathogens consisting essentially of a core of a single nucleic acid surrounded by a protein coat. The smallest known infectious organism.
Characterized by their inability to reproduce outside of a living host cell. Viruses may subvert the host cells' normal functions, causing the cell to behave in a manner determined by the virus. They are unable to live or multiply outside of a host cell, since most do not possess the means to synthesize protein.

Once inside a cell, a virus releases its DNA or RNA (the genetic code to produce more viruses) and takes over some aspects of the host cell's metabolism. The virus then uses its host to manufacture more virus particles. The virus either kills its host or alters it so that it divides abnormally, becoming cancerous. Viruses are usually cell- and, to a certain extent, species-specific.

Defense against viruses include the physical barriers of skin and mucous membranes as well as interferon , which is released by infected cells to increase the resistance of surrounding nonifected cells. The last line of defense are the various types of white blood cell . Immunity to a virus is produced by injecting a vaccine that resembles a virus, thus increasing the number of T and B-cells that can recognise the virus.