ETYM Latin, a slimy liquid, a poisonous liquid, poison, stench.
Infectious particle consisting of a core of nucleic acid (DNA or RNA) enclosed in a protein shell. Viruses are acellular and able to function and reproduce only if they can invade a living cell to use the cell's system to replicate themselves. In the process they may disrupt or alter the host cell's own DNA. The healthy human body reacts by producing an antiviral protein, interferon, which prevents the infection spreading to adjacent cells.
Many viruses mutate continuously so that the host’s body has little chance of developing permanent resistance; others transfer between species, with the new host similarly unable to develop resistance. The viruses that cause AIDS and Lassa fever are both thought to have “jumped” to humans from other mammalian hosts.
Among diseases caused by viruses are canine distemper, chickenpox, common cold, herpes, influenza, rabies, smallpox, yellow fever, AIDS, and many plant diseases. Recent evidence implicates viruses in the development of some forms of cancer (see oncogenes). Bacteriophages are viruses that infect bacterial cells.
Retroviruses are of special interest because they have an RNA genome and can produce DNA from this RNA by a process called reverse transcription.
Viroids, discovered 1971, are even smaller than viruses; they consist of a single strand of nucleic acid with no protein coat. They may cause stunting in plants and some rare diseases in animals, including humans. It is debatable whether viruses and viroids are truly living organisms, since they are incapable of an independent existence. Outside the cell of another organism they remain completely inert. The origin of viruses is also unclear, but it is believed that they are degenerate forms of life, derived from cellular organisms, or pieces of nucleic acid that have broken away from the genome of some higher organism and taken up a parasitic existence.
Antiviral drugs are difficult to develop because viruses replicate by using the genetic machinery of host cells, so that drugs tend to affect the host cell as well as the virus. Acyclovir (used against the herpes group of diseases) is one of the few drugs so far developed that is successfully selective in its action. It is converted to its active form by an enzyme that is specific to the virus, and it then specifically inhibits viral replication. Some viruses have shown developing resistance to the few antiviral drugs available. Viruses have recently been found to be very abundant in seas and lakes, with between 5 and 10 million per milliliter of water at most sites tested, but up to 250 million per milliliter in one polluted lake. These viruses infect bacteria and, possibly, single-celled algae. They may play a crucial role in controlling the survival of bacteria and algae in the plankton.
(Irregular plural: viruses).
Ultramicroscopic infectious agent that replicates itself only within cells of living hosts; many are pathogenic.

Malicious code that infects files on your system.
An intrusive program that infects computer files by inserting in those files copies of itself. The copies are usually executed when the file is loaded into memory, allowing the virus to infect still other files, and so on. Viruses often have damaging side effects—sometimes intentionally, sometimes not. For example, some viruses can destroy a computer’s hard disk or take up memory space that could otherwise be used by programs. See also Good Times virus, Trojan horse, worm.
In computing, a piece of software that can replicate and transfer itself from one computer to another, without the user being aware of it. Some viruses are relatively harmless, but others can damage or destroy data. They are written by anonymous programmers, often maliciously, and are spread along telephone lines or on floppy discs. Antivirus software can be used to detect and destroy well-known viruses, but new viruses continually appear and these may bypass existing antivirus programs.
Computer viruses may be programmed to operate on a particular date, such as the Michelangelo Virus, which was triggered on 6 March 1992 (the anniversary of the birthday of Italian artist Michelangelo) and erased hard discs.