Portal:Viruses
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
Aedes aegypti can transmit the chikungunya, dengue, yellow fever and Zika viruses. The mosquito is widespread in tropical and subtropical regions, with mosquito control being key to disease prevention.
Credit: United States Department of Agriculture (2000)
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
Virus classification is the process of naming viruses and placing them into a taxonomic system. They are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms and the type of disease they cause.
Two schemes are in common use. The International Committee on Taxonomy of Viruses (ICTV), established in the early 1970s, classifies viruses into taxa (groups) similar to the biological classification used for cellular organisms, which reflect viruses believed to have a common ancestor. As of 2019, 9 kingdoms, 16 phyla, 36 classes, 55 orders, 168 families, 1,421 genera and 6,589 species of viruses have been defined. Since 2018, viruses have also been classified into higher-level taxa called realms. Four realms are defined, as of 2020, encompassing almost all RNA viruses; some DNA viruses have yet to be assigned a realm.
The older Baltimore classification (pictured), proposed in 1971 by David Baltimore, places viruses into seven groups (I–VII) based on their nucleic acid type, number of strands and sense, as well as the method the virus uses to generate mRNA. There is some concordance between Baltimore groups and the higher levels of the ICTV scheme.
Selected outbreak
The 2001 foot-and-mouth outbreak included 2,000 cases of the disease in cattle and sheep across the UK. The source was a Northumberland farm where pigs had been fed infected meat that had not been adequately sterilised. The initial cases were reported in February. The disease was concentrated in western and northern England, southern Scotland and Wales, with Cumbria being the worst-affected area. A small outbreak occurred in the Netherlands, and there were a few cases elsewhere in Europe.
The UK outbreak was controlled by the beginning of October. Control measures included stopping livestock movement and slaughtering over 6 million cows and sheep. Public access to farmland and moorland was also restricted (pictured), greatly reducing tourism in affected areas, particularly in the Lake District. Vaccination was used in the Netherlands, but not in the UK due to concerns that vaccinated livestock could not be exported. The outbreak cost an estimated £8 billion in the UK.
Selected quotation
| “ | We are part virus. This bizarre yet inescapable fact has been revealed over the past 30 years, as scientists have spelunked their way through the human genome and encountered stretches of DNA with the telltale chemical signatures of viruses. | ” |
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
Canine parvovirus type 2 (CPV2) is a non-enveloped, single-stranded DNA virus in the Parvoviridae family. The icosahedral viral capsid is only 20–26 nm in diameter, making it one of the smallest viruses. The genome is about 5000 nucleotides long. The virus is very similar to feline panleukopenia virus, another parvovirus, as well as mink enteritis and raccoon and fox parvoviruses. It infects dogs, wolves, foxes and other canids, big cats and occasionally domestic cats, but cannot infect humans.
A relatively new disease, CPV2 infection was first recognised in 1978 and rapidly spread worldwide. The virus is stable and highly infectious, being transmitted by contact with faeces, infected soil or contaminated objects. After ingestion, the virus replicates in the lymphoid tissue in the throat, then spreads to the bloodstream to infect cells of the lymph nodes, intestinal crypts and bone marrow, damaging the intestinal lining. The more common intestinal form of disease causes vomiting and severe, often bloody diarrhoea. The cardiac form affects puppies under 8 weeks, causing respiratory or cardiovascular failure; mortality can reach 91% in untreated cases. No specific antiviral drug is available. Prevention is by vaccination.
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
Ryan Wayne White (6 December 1971 – 8 April 1990) was an HIV-positive American teenager who became a national spokesman for AIDS research and public education about HIV/AIDS, after being expelled from school because of his infection.
White, a haemophiliac, was diagnosed in 1984 after infection by a contaminated blood treatment. HIV/AIDS was then poorly understood, and his return to school in Kokomo, Indiana was prevented by protesters; the ensuing legal battle gained national media coverage. Before his case, AIDS was widely associated with the male gay community; White was one of several who helped to shift that perception.
White died in 1990, one month before his high school graduation. Shortly afterwards, the U.S. Congress passed a major piece of AIDS legislation, the Ryan White Care Act. Ryan White Programs remain the largest provider of services for people living with HIV/AIDS in the United States.
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
Two polio vaccines are used against the paralytic disease polio. The first, developed by Jonas Salk, consists of inactivated poliovirus. Based on three wild virulent strains, inactivated using formalin, it is administered by injection and is very safe. It confers IgG-mediated immunity, which prevents poliovirus from entering the bloodstream and protects the motor neurons, eliminating the risk of bulbar polio and post-polio syndrome. The second, developed by Albert Sabin, originally consisted of three live virus strains, attenuated by growth in cell culture. Since 2016, only two strains have generally been included. They contain multiple mutations, preventing them from replicating in the nervous system. The Sabin vaccine stimulates both antibodies and cell-mediated immunity, providing longer-lasting immunity than the Salk vaccine. It can be administered orally, making it more suitable for mass vaccination campaigns. In around three cases per million doses, the live vaccine reverts to a virulent form and causes paralysis. Vaccination has reduced the number of wild-type polio cases from around 350,000 in 1988 to just 33 in 2018, and eradicated the disease from most countries.
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