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A '''virus''' (from the [[Latin]] virus meaning toxin or [[poison]]) is a microscopic infectious agent that can reproduce only inside a host [[cell]]. Viruses infect all types of [[organism]]s: from [[anima]]ls and plants, to bacteria and archaea.[1] Since the initial [[discovery]] of tobacco mosaic virus by [http://en.wikipedia.org/wiki/Martinus_Beijerinck Martinus Beijerinck] in 1898,[2] more than 5,000 types of virus have been described in detail,[3] although most types of virus remain undiscovered.[4] Viruses are ubiquitous, as they are found in almost every [[ecosystem]] on [[Earth]],[5] and are the most abundant type of [[biological]] entity on the planet.[6] The study of viruses is known as virology, and is a branch of microbiology.

A '''virus''' (from the [[Latin]] virus meaning toxin or [[poison]]) is a microscopic infectious agent that can reproduce only inside a host [[cell]]. Viruses infect all types of [[organism]]s: from [[anima]]ls and plants, to bacteria and archaea.[1] Since the initial [[discovery]] of tobacco mosaic virus by [https://en.wikipedia.org/wiki/Martinus_Beijerinck Martinus Beijerinck] in 1898,[2] more than 5,000 types of virus have been described in detail,[3] although most types of virus remain undiscovered.[4] Viruses are ubiquitous, as they are found in almost every [[ecosystem]] on [[Earth]],[5] and are the most abundant type of [[biological]] entity on the planet.[6] The study of viruses is known as virology, and is a branch of microbiology.

Viruses consist of two or three parts: all viruses have [[genes]] made from either [[DNA]] or [[RNA]], long molecules that carry genetic [[information]]; all have a [[protein]] coat that protects these genes; and some have an envelope of fat that surrounds them when they are outside a cell. Viruses vary in shape from simple [[helical]] and [[icosahedral]] shapes, to more [[complex]] [[structure]]s. They are about 1/100th the size of bacteria.[7] The [[origin]]s of viruses in the [[evolution]]ary [[history]] of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic [[diversity]].[8]

Viruses consist of two or three parts: all viruses have [[genes]] made from either [[DNA]] or [[RNA]], long molecules that carry genetic [[information]]; all have a [[protein]] coat that protects these genes; and some have an envelope of fat that surrounds them when they are outside a cell. Viruses vary in shape from simple [[helical]] and [[icosahedral]] shapes, to more [[complex]] [[structure]]s. They are about 1/100th the size of bacteria.[7] The [[origin]]s of viruses in the [[evolution]]ary [[history]] of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic [[diversity]].[8]

Viruses spread in many ways; plant viruses are often transmitted from plant to plant by [[insects]] that feed on sap, such as aphids, while [[animal]] viruses can be carried by blood-sucking insects. These [[disease]]-bearing [[organism]]s are known as [[vectors]]. Influenza viruses are spread by coughing and sneezing, and others such as [http://en.wikipedia.org/wiki/Norovirus norovirus], are transmitted by the faecal-[[oral]] route, when they contaminate hands, food, or water. [http://en.wikipedia.org/wiki/Rotavirus Rotaviruses] are often spread by direct contact with infected children. HIV is one of several viruses that are transmitted through [[sexual]] contact.

Viruses spread in many ways; plant viruses are often transmitted from plant to plant by [[insects]] that feed on sap, such as aphids, while [[animal]] viruses can be carried by blood-sucking insects. These [[disease]]-bearing [[organism]]s are known as [[vectors]]. Influenza viruses are spread by coughing and sneezing, and others such as [https://en.wikipedia.org/wiki/Norovirus norovirus], are transmitted by the faecal-[[oral]] route, when they contaminate hands, food, or water. [https://en.wikipedia.org/wiki/Rotavirus Rotaviruses] are often spread by direct contact with infected children. HIV is one of several viruses that are transmitted through [[sexual]] contact.

Not all viruses cause disease, as many viruses reproduce without causing any obvious harm to the infected [[organism]]. Viruses such as hepatitis B can cause life-long or chronic infections, and the viruses continue to replicate in the [[body]] despite the hosts' defense [[mechanism]]s. In some cases, these chronic infections might be beneficial as they might increase the immune system's response against infection by other pathogens.[9] However, in most cases viral infections in animals cause an immune response that eliminates the infecting virus. These immune responses can also be produced by vaccines that give lifelong immunity to a viral infection. Microorganisms such as bacteria also have defenses against viral infection, such as restriction modification [[system]]s. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat both life-threatening and more minor infections.[http://en.wikipedia.org/wiki/Virus]

Not all viruses cause disease, as many viruses reproduce without causing any obvious harm to the infected [[organism]]. Viruses such as hepatitis B can cause life-long or chronic infections, and the viruses continue to replicate in the [[body]] despite the hosts' defense [[mechanism]]s. In some cases, these chronic infections might be beneficial as they might increase the immune system's response against infection by other pathogens.[9] However, in most cases viral infections in animals cause an immune response that eliminates the infecting virus. These immune responses can also be produced by vaccines that give lifelong immunity to a viral infection. Microorganisms such as bacteria also have defenses against viral infection, such as restriction modification [[system]]s. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat both life-threatening and more minor infections.[https://en.wikipedia.org/wiki/Virus]

[[Category: Biology]]

[[Category: Biology]]