A virus is a small infectious agent that can replicate only inside the living cells of an organism. Viruses can infect all types of organisms, from animals and plants to bacteria and archaea.
Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants, and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, about 5,000 viruses have been described in detail, although there are millions of different types. Viruses are found in almost every ecosystem on Earth and are the most abundant type of biological entity. The study of viruses is known as virology, a sub-speciality of microbiology.
Virus particles (known as virions) consist of two or three parts: the genetic material made from either DNA or RNA, long molecules that carry genetic information; a protein coat that protects these genes; and in some cases an envelope of lipids that surrounds the protein coat when they are outside a cell. The shapes of viruses range from simple helical and icosahedral forms to more complex structures. The average virus is about one one-hundredth the size of the average bacterium. Most viruses are too small to be seen directly with an optical microscope.
The origins of viruses in the evolutionary 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.
Viruses spread in many ways; viruses in plants are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; viruses in animals can be carried by blood-sucking insects. These disease-bearing organisms are known as vectors. Influenza viruses are spread by coughing and sneezing. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route and are passed from person to person by contact, entering the body in food or water. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. The range of host cells that a virus can infect is called its "host range". This can be narrow or, as when a virus is capable of infecting many species, broad.
Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. However, some viruses including those that cause AIDS and viral hepatitis evade these immune responses and result in chronic infections. Antibiotics have no effect on viruses, but several antiviral drugs have been developed.
Other articles related to "virus":
... The B19 virus, generally referred to as parvovirus B19 or sometimes erythrovirus B19, was the first (and until 2005 the only) known human virus in the family of parvoviruses, genus erythrovirus ... B19 virus causes a childhood rash called fifth disease or erythema infectiosum which is commonly called slapped cheek syndrome ... The virus was discovered by chance in 1975 by Australian virologist Yvonne Cossart ...
... Paget's disease may be caused by a slow virus infection (i.e ... Canine distemper virus Respiratory syncytial virus Contamination has been suggested to have played a role in producing past positive results ...
... Chikungunya virus is indigenous to tropical Africa and Asia, where it is transmitted to humans by the bite of infected mosquitoes, usually of the genus Aedes ... Chikungunya virus belongs to alphavirus genus of the Togaviridae family ... The main virus reservoirs are monkeys, but other species can also be affected, including humans ...
... the successful recreation of the infamous 1918 influenza virus in a laboratory ... The smallpox virus devastated numerous societies throughout history before its eradication ... in the world that keep stocks of smallpox virus – the Russian Vector laboratory, and the United States Centers for Disease Control ...
... scabiei Flavivirus Infections Yellow fever virus, West Nile virus, dengue virus, Tick-borne encephalitis virus Some tropical diseases are very rare, but may occur in sudden epidemics, such as the Ebola ...
Famous quotes containing the word virus:
“If I knew for a certainty that a man was coming to my house with the conscious design of doing me good, I should run for my life ... for fear that I should get some of his good done to me,some of its virus mingled with my blood.”
—Henry David Thoreau (18171862)
“Think of the earth as a living organism that is being attacked by billions of bacteria whose numbers double every forty years. Either the host dies, or the virus dies, or both die.”
—Gore Vidal (b. 1925)
“Freedom is the most contagious virus known to man.”
—Hubert H. Humphrey (19111978)