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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 Viral replication 7 Origins 8 Human viral diseases 9 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

• 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).

Viral structure

Viruses: non-living or alive?

Giant viruses

Viral replication

Origins

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

Human viral diseases

Prevention and treatment of viral diseases

See also: Computer virus, Nanobacterium

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