The Viruses Portal
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Viruses are small infectious agents that can replicate only inside the living cells of an organism. Viruses infect all forms of life, including animals, plants, fungi, bacteria and archaea. They are found in almost every ecosystem on Earth and are the most abundant type of biological entity, with millions of different types, although only about 6,000 viruses have been described in detail. Some viruses cause disease in humans, and others are responsible for economically important diseases of livestock and crops.
Virus particles (known as virions) consist of genetic material, which can be either DNA or RNA, wrapped in a protein coat called the capsid; some viruses also have an outer lipid envelope. The capsid can take simple helical or icosahedral forms, or more complex structures. The average virus is about 1/100 the size of the average bacterium, and most are too small to be seen directly with an optical microscope.
The origins of viruses are unclear: some may have evolved from plasmids, others from bacteria. Viruses are sometimes considered to be a life form, because they carry genetic material, reproduce and evolve through natural selection. However they lack key characteristics (such as cell structure) that are generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life".
Selected disease
Foot-and-mouth disease or FMD is an economically important disease of even-toed ungulates (cloven-hoofed animals) and some other mammals caused by the FMD virus, a picornavirus. Hosts include cattle, water buffalo, sheep, goats, pigs, antelope, deer and bison; human infection is extremely rare. After a 1–12-day incubation, animals develop high fever, and then blisters inside the mouth (pictured) and on the hooves, which can rupture and cause lameness. Weight loss and reduction in milk production are other possible long-term consequences. Mortality in adult animals is low (2–5%). The virus is highly infectious, with transmission occurring via direct contact, aerosols, semen, consumption of infected food scraps or feed supplements, and via inanimate objects including fodder, farming equipment, vehicles, standing water, and the clothes and skin of humans. Some infected ruminants can transmit infection as asymptomatic carriers.
Friedrich Loeffler showed the disease to be viral in 1897. FMD was widely distributed in 1945. By 2014, North America, Australia, New Zealand, much of Europe, and some South American countries were free of the disease. Major outbreaks include one in the UK in 2001 that cost an estimated £8 billion. The virus is highly variable, with seven serotypes. A vaccine is available, but protection is temporary and strain specific. Other control methods include monitoring programmes, trade restrictions, quarantine, and the slaughter of infected and healthy at-risk animals.
Selected image
False-coloured transmission electron micrograph of severe acute respiratory syndrome coronavirus 2, a novel coronavirus, showing the spikes (blue) forming a crown that give this group of RNA viruses their name. The spike protein interacts with the cellular receptor for the virus, angiotensin-converting enzyme 2.
Credit: National Institute of Allergy and Infectious Diseases (13 February 2020)
In the news
26 February: In the ongoing pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), more than 110 million confirmed cases, including 2.5 million deaths, have been documented globally since the outbreak began in December 2019. WHO
18 February: Seven asymptomatic cases of avian influenza A subtype H5N8, the first documented H5N8 cases in humans, are reported in Astrakhan Oblast, Russia, after more than 100,0000 hens died on a poultry farm in December. WHO
14 February: Seven cases of Ebola virus disease are reported in Gouécké, south-east Guinea. WHO
7 February: A case of Ebola virus disease is detected in North Kivu Province of the Democratic Republic of the Congo. WHO
4 February: An outbreak of Rift Valley fever is ongoing in Kenya, with 32 human cases, including 11 deaths, since the outbreak started in November. WHO
21 November: The US Food and Drug Administration (FDA) gives emergency-use authorisation to casirivimab/imdevimab, a combination monoclonal antibody (mAb) therapy for non-hospitalised people twelve years and over with mild-to-moderate COVID-19, after granting emergency-use authorisation to the single mAb bamlanivimab earlier in the month. FDA 1, 2
18 November: The outbreak of Ebola virus disease in Équateur Province, Democratic Republic of the Congo, which started in June, has been declared over; a total of 130 cases were recorded, with 55 deaths. UN
Selected article
Viruses and viral infections have affected human history. Epidemics caused by viruses began when human behaviour changed during the Neolithic period, around 12,000 years ago. Previously hunter-gatherers, humans developed more densely populated agricultural communities, which allowed viruses to spread rapidly and subsequently to become endemic. Viruses of plants and livestock also increased, and as humans became dependent on agriculture and farming, diseases such as potyviruses of potatoes and rinderpest of cattle had devastating consequences. Smallpox and measles viruses are among the oldest that infect humans. They first appeared in humans in Europe and North Africa thousands of years ago, having evolved from viruses that infected other animals. Influenza pandemics have been recorded since 1580.
There are an estimated 1031 viruses on Earth. Although scientific interest in them arose because of the diseases they cause, most viruses are beneficial. They drive evolution by transferring genes across species, play important roles in ecosystems, and are essential to life.
Selected outbreak
In the severe acute respiratory syndrome (SARS) outbreak, the first cases of the newly emerged SARS coronavirus were reported in November 2002 from the Chinese Guangdong province. The virus soon spread across Asia, with China, Hong Kong, Taiwan and Singapore being the worst affected countries; a secondary outbreak occurred in Canada. The rapid initial spread of the outbreak has been in part attributed to China's slow response to the early cases. Over 8,000 people were infected, with a case fatality rate of 11%. Those over 65 years had a much higher mortality rate, greater than 55%. The outbreak was contained by July 2003, and no cases have been reported since 2004.
At the time of the outbreak, the immediate source of SARS coronavirus was thought to have been the masked palm civet (Paguma larvata; pictured), which was sold as food in Guangdong markets. The virus was also found in raccoon dogs, ferret badgers and domestic cats. More recent research has suggested that the natural reservoir could be horseshoe bats.
Selected quotation
“ | There is no question that virology has long outgrown the confines of "Microbiology": it is an important discipline in its own right, a fact that is attested by the numerous high quality journals that are exclusively devoted to it. Further, it is primarily virology that has made possible the rapid rise of molecular cell biology. | ” |
—Bill Joklik on founding a society for virology
Recommended articles
Viruses & Subviral agents: bat virome • elephant endotheliotropic herpesvirus • HIV • introduction to viruses • Playa de Oro virus • poliovirus • prion • rotavirus • virus
Diseases: colony collapse disorder • common cold • croup • dengue fever • gastroenteritis • Guillain–Barré syndrome • hepatitis B • hepatitis C • hepatitis E • herpes simplex • HIV/AIDS • influenza • meningitis • myxomatosis • polio • pneumonia • shingles • smallpox
Epidemiology & Interventions: 2007 Bernard Matthews H5N1 outbreak • Coalition for Epidemic Preparedness Innovations • Disease X • 2009 flu pandemic • HIV/AIDS in Malawi • polio vaccine • Spanish flu • West African Ebola virus epidemic
Virus–Host interactions: antibody • host • immune system • parasitism • RNA interference
Methodology: metagenomics
Social & Media: And the Band Played On • Contagion • "Flu Season" • Frank's Cock • Race Against Time: Searching for Hope in AIDS-Ravaged Africa • social history of viruses • "Steve Burdick" • "The Time Is Now" • "What Lies Below"
People: Brownie Mary • Macfarlane Burnet • Bobbi Campbell • Aniru Conteh • people with hepatitis C • HIV-positive people • Bette Korber • Henrietta Lacks • Linda Laubenstein • Barbara McClintock • poliomyelitis survivors • Joseph Sonnabend • Eli Todd • Ryan White
Selected virus
Tobacco mosaic virus (TMV) is an RNA virus in the Virgaviridae family that infects a wide range of plants, including tobacco, tomato, pepper, other members of the Solanaceae family, and cucumber. The rod-shaped virus particle is around 300 nm long and 18 nm in diameter, and consists of a helical capsid made from 2130 copies of a single coat protein, which is wrapped around a positive-sense single-stranded RNA genome of around 6400 bases. The coat protein and RNA can self-assemble to produce infectious virus.
Infection often causes characteristic patterns, such as "mosaic"-like mottling and discoloration on the leaves, but is almost symptomless in some host species. TMV causes an economically important disease in tobacco plants. Transmission is frequently by human handling, and prevention of infection involves destroying infected plants, hand washing and crop rotation to avoid contaminated soil. TMV is one of the most stable viruses known. The fact that it does not infect animals and can readily be produced in gramme amounts has led to its use in numerous pioneering studies in virology and structural biology. TMV was the first virus to be discovered and the first to be crystallised.
Did you know?
- ...that the elegant rice rat (pictured; top) sometimes carries a hantavirus that can cause a fatal disease in humans?
- ...that a quaranjavirus that can infect humans was discovered in 1953, but it took 60 years to classify it?
- ...that in 1918, infected crew members aboard HMS Mantua inadvertently spread the Spanish flu to Africa?
- ...that in the mid-1980s, some HIV patients pinned their hopes for survival on an experimental drug called HPA-23?
- ...that Li Zaiping and his research group were the first to sequence a viral genome in China?
Selected biography
Walter Reed (13 September 1851 – 22 November 1902) was an American physician in the U.S. Army medical corps who is known for his research on the epidemiology of yellow fever, at a time when viruses had only just been discovered.
Reed started to study yellow fever in the USA in the 1890s, showing that walking through swampy woods at night was associated with the disease, while drinking water from the Potomac River was not. In 1900, he led an army commission under the direction of George Miller Sternberg to investigate yellow fever in Cuba, where the disease was endemic. Building on Carlos Finlay's work suggesting that yellow fever was transmitted by a particular species of mosquito acting as a vector, Reed and co-workers confirmed Finlay's results, and also disproved the popular idea that the disease was transmitted by contaminated objects, such as clothing or bedding. The experiments involved the deliberate infection of human volunteers, several of whom died of yellow fever, and Reed pioneered the concept of medical consent.
In this month
5 June 1981: First report of HIV/AIDS (symbol pictured) appeared in medical literature
6 June 1997: Gene silencing in plants shown to be a viral defence mechanism
7–13 June 1962: Donald Caspar and Aaron Klug proposed the quasi-equivalence principle of virus structure
7–13 June 1962: André Lwoff proposed a viral classification scheme based on nature of genome, type of symmetry and presence of envelope
7–13 June 1962: George Hirst proposed that the influenza virus genome is segmented
9 June 1981: The American Society for Virology was founded
13 June 2012: First case of Middle East respiratory syndrome coronavirus (MERS-CoV) occurred in Saudi Arabia
18 June 1981: A vaccine against foot-and-mouth disease was the first genetically engineered vaccine
21 June 1996: Nevirapine approved, first NNRTI for HIV/AIDS
26 June 1993: Clinical trial of hepatitis B virus drug fialuridine terminated; the drug caused several fatalities due to lactic acidosis
28 June 2011: FAO declared rinderpest eradicated
30 June 1985: Ryan White was denied re-admittance to his school after an AIDS diagnosis, in a case that changed public perceptions of the disease
Selected intervention
Influenza vaccines include live attenuated and inactivated forms. Inactivated vaccines contain three or four different viral strains selected by the World Health Organization to cover influenza A H1N1 and H3N2, as well as influenza B, and are usually administered by intramuscular injection. The live attenuated influenza vaccine contains a cold-adapted strain and is given as a nasal spray. Most influenza vaccine strains are cultivated in fertilised chicken eggs (pictured), a technique developed in the 1950s; others are grown in cell cultures, and some vaccines contain recombinant proteins. Annual vaccination is recommended for high-risk groups and, in some countries, for all those over six months. As the influenza virus changes rapidly by antigenic drift, new versions of the vaccine are developed twice a year, which differ in effectiveness depending on how well they match the circulating strains. Despite considerable research effort for decades, no effective universal influenza vaccine has been identified. A 2018 meta-analysis found that vaccination in healthy adults decreased confirmed cases of influenza from about 2.4% to 1.1%. However, the effectiveness is uncertain in those over 65 years old, one of the groups at highest risk of serious complications.
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