Virus

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A common alternate meaning of virus is computer virus. Other meanings, as well as a discussion of pluralization, are at plural of virus

A virus is a small particle which can infect other biological organisms. Viruses are obligate intracellular parasites meaning that they can only reproduce by invading and taking over other cells as they lack the cellular machinery for self reproduction. The term virus usually refers to those particles which infect eukaryotes (multi-celled organisms and many single-celled organisms), whilst the term 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.

Table of contents

1 Etymology
2 Virus classifications
3 Viral structure

3.1 The viral envelope
3.2 Capsid
3.3 Viral genome

4 Viruses: non-living or alive?
5 Giant viruses
6 Origins
7 Human viral diseases
8 Prevention and treatment of viral diseases

Etymology

The word comes from the Latin virus, referring to poison and other noxious things. Today it is used to describe the biological viruses discussed above and also as a metaphor for other parasitically-reproducing things, such as ideas. The term computer virus has become another well-defined sense of the word. The word virion or viron is used to refer to a single infective viral particle.

Despite frequent claims to the contrary, the only correct English plural of the word for any of these senses is viruses. The Latin word does not appear to have had a plural. Virii would be the plural of the word virius, and viri was the plural of the word vir, meaning man. See [1] for more on this.

Virus classifications

There are several different viral classifications. The following is a typical example:

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 (retroviruses).
Class VII double stranded DNA with an RNA intermediate in replication (Hepadnaviruses).

It should be noted that none of these classifications are expected to be phylogenetic, as viruses may not share a common origin.

Viral structure

Viruses typically consist of a protein coat (the envelope), a protein core (the capsid) that encloses the viral genes and the viral genetic material itself. The envelope, normally derived from the cell membrane of the previous host, protects the viral genome contained within and also provides the mechanism by which the virus infects its host.

Viruses: non-living or alive?

A virus hijacks its host's cell machinery to create more 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", 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.

Giant viruses

Some viruses are quite large, particularly some that exist as metabolic parasites inside host cells. A giant intracellular virus, Mimivirus, survives inside amoebae that can be found in the water of cooling towers. Mimivirus has a DNA genome of about 800 kilobase pairs, larger than the genomes of several bacteria.

Origins

The origin of viruses is not entirely clear, but the currently favoured explanation is that they are derived from their host organisms, originating from transferrable elements like plasmids or transposons. It has also been suggested that they may represent extremely reduced microbes, appeared separately in the primordial soup that gave rise to the first cells, or that the different sorts of viruses appeared through different mechanisms.

Other infectious particles which are even simpler in structure than viruses include viroids, virusoids, and prions.

Human viral diseases

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. The relative ability of viruses to cause disease is described in terms of virulence.

Prevention and treatment of viral diseases

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.

See also: Computer virus