Virus

Alternate meanings: computer virus and see virus (disambiguation)

A virus (from the Latin virus, referring to poison and other noxious things) is a small particle which can infect other biological organisms. The term "virus" usually refers to those particles which infect eukaryotes (multi-celled organisms and many single-celled organisms), whilst "bacteriophage" or "phage" is used to describe those infecting prokaryotes (bacteria and bacteria-like organisms). Typically these particles carry a small amount of nucleic acid (either DNA or RNA) surrounded by some form of protective "coat" consisting of protein, or protein and lipid.

One of several possible viral classifications:

Class I double stranded DNA.
Class II single stranded DNA.
Class III double stranded RNA.
Class IV positive single stranded RNA itself acting as mRNA.
Class V negative single stranded RNA used as a template for mRNA synthesis.
Class VI positive single stranded RNA with a DNA intermediate in
Class VII double stranded DNA with an RNA intermediate in replication.

The protective coat normally also enables the infective process which can occur by a variety of different mechanisms. The practical upshot of all of these is that the host cell's replication machinery is hijacked to create more of the virus particles, hence completing the life cycle. Viruses are somewhere between being living and non-living. They can reproduce and show inheritance, but are reliant on the complex enzymes of their hosts, and in many ways can be treated like ordinary molecules (for instance, they can be crystalized). Whether or not they are "alive," it is clear that they are obligate parasites, and have no form which can reproduce independent of their host. Like most parasites they have a specific host range, sometimes specific to one species (or even limited cell types of one species) and sometimes more general.

Examples of diseases caused by viruses include the common cold, which is caused by a variety of related viruses; smallpox; AIDS, which is caused by HIV; and cold sores, which are caused by herpes simplex. Recently it has been shown that cervical cancer is caused at least partly by papillomavirus (which causes papillomas, or warts), representing the first significant evidence in humans for a link between cancer and an infective agent.

Because they use the machinery of their host cells, viruses are difficult to kill. The most effective medical approaches to viral diseases, thus far, are vaccination to prevent infection, and drugs that treat the symptoms of viral infections. Patients often ask for antibiotics, which are useless against viruses, and their misuse against viral infections is one of the causes of antibiotic resistance in bacteria. That said, sometimes the prudent course of action is to begin a course of antibiotic treatment while waiting for test results to determine whether the patient's symptoms are caused by a virus or a bacterial infection.

Derivation

Derived from the Latin virus (meaning roughly "poison") the word was first used for the discovery of infectious disease-causing agents smaller than bacteria that reproduce with the help of a host cell. It is also used metaphorically to refer to other parasitically-reproducing things like ideas. The computer science term virus, once metaphorical, is now another well-defined sense of the word.

Despite frequent claims to the contrary, the only correct English plural of the word used in any of these senses is viruses, not virii. The "ii" is used to denote plurality in Latin words ending in "ius", not "us." Hence it is "viruses", not "virii." If virus was spelled "virius," then the plural "virii" would be correct, however, it is not. See, for example, [1].

The word virion is used to refer to a single infective viral particle.