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
Dengue fever, or breakbone fever, is an infectious tropical disease caused by the dengue virus. Symptoms include fever, headache, muscle and joint pains, and a characteristic skin rash that is similar to measles. Rarely, the disease develops into the life-threatening dengue haemorrhagic fever, resulting in bleeding, low levels of blood platelets and blood plasma leakage, or into dengue shock syndrome, where dangerously low blood pressure occurs.
Dengue is transmitted by several species of Aedes mosquito, mainly A. aegypti. First described in 1779, the incidence of dengue fever has increased dramatically since the Second World War, with around 390 million people infected each year, of whom about 40,000 die. The virus is endemic in more than 120 countries, mainly in Southeast Asia, South Asia and South America. Infection with one of the five viral types usually gives lifelong immunity to that type but only short-term immunity to the others, and subsequent infection with a different type increases the risk of severe complications. Treatment of acute dengue is supportive, using oral or intravenous fluids for mild or moderate disease; blood transfusion may be required in more severe cases. A vaccine against dengue fever is available. Reducing mosquito habitats and limiting exposure to bites are the other main methods of control.
Selected image
HIV-1 budding from lymphocytes in culture. HIV establishes a latent infection in several types of immune cell and causes profound immunodeficiency.
Credit: C. Goldsmith (1984)
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
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Bats host a diverse array of viruses, including all seven types described by the Baltimore classification system. The most common viruses known to infect bats are coronaviruses. Bats harbour many viruses that are zoonotic, or capable of infecting humans, including rabies virus, SARS-CoV, SARS-CoV-2, MERS-CoV, Nipah virus, Hendra virus and Marburg virus (hosted by the Egyptian fruit bat; pictured), and some bat-borne viruses are considered important emerging viruses. Bats may also play a role in the ecology of the Ebola virus. Most zoonotic bat viruses are transmitted by direct contact with infected bat fluids such as urine, guano and saliva, or through contact with an infected intermediate host; transmission of rabies from bats to humans usually occurs via biting. Butchering or consuming bat meat could potentially lead to viral transmission.
Bats rarely become ill from viral infections, and rabies is the only viral disease known to kill them. They might be more tolerant of infection than other mammals. Their immune systems differ from those of other mammals in their lack of several inflammasomes, which activate the body's inflammatory response, as well as a dampened stimulator of interferon genes response, which helps to control the host response to pathogens.
Selected outbreak
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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
| “ | The unusual features of the giant Mimivirus revived the popular, yet unresolved question: "Are viruses alive?" The discovery that some of them can get sick adds a new twist to this old debate. | ” |
—Hiroyuki Ogata & Jean-Michel Claverie on the relationship between Sputnik virophage and mimivirus
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
Sputnik virophage is a subviral agent, discovered in 2008, that infects Acanthamoeba protozoa. It is a satellite virus of giant viruses of the Mimiviridae family. It requires a mimivirus to infect the cell simultaneously to replicate, hijacking the virus factories that mimivirus creates and impairing its replication. Sputnik was the first satellite to be shown to inhibit the replication of its associated helper virus. Such viruses have been termed "virophages" or "virus eaters" – by analogy with bacteriophages, viruses that parasitise bacteria – but the distinction between virophages and classical satellite viruses that infect plants, arthropods and mammals is disputed. Three Sputnik types are now known, and other virophages have since been discovered, now classified in the Lavidaviridae family, including the Zamilon, Mavirus and Organic Lake virophages. All virophages that have been characterised infect protists and all rely on nucleocytoplasmic large DNA viruses as helpers.
Sputnik's non-enveloped icosahedral capsid is 74 nm in diameter, and contains a circular double-stranded DNA genome of 18.3 kb. Three of its 21 predicted protein-coding genes are thought to derive from Acanthamoeba polyphaga mimivirus, suggesting that virophages and giant viruses can swap genes during their joint infection of Acanthamoeba, and also that virophages might mediate horizontal gene transfer between giant viruses.
Did you know?
- ...that the protein ASF/SF2 (molecular image pictured) is involved in the development of the human heart and the replication of HIV-1?
- ...that in February 2020, Neil Ferguson and his team believed that significantly more people in China had been infected with the 2019 novel coronavirus than had been reported?
- ...that the outbreak of Ebola in Bundibugyo District in Uganda in 2007 was caused by a novel strain of the Ebola virus?
- ...that virologist John R. Paul blamed better hygiene for polio's spread in the 20th century, saying early exposure to poliovirus would have given immunity?
- ...that a novel human polyomavirus is associated with Merkel cell carcinoma, a rare and highly aggressive form of skin cancer?
Selected biography
Rosalind Elsie Franklin (25 July 1920 – 16 April 1958) was a British biophysicist and X-ray crystallographer who made critical contributions to the understanding of the fine molecular structures of DNA, RNA and viruses.
Franklin led pioneering research on the structure of tobacco mosaic virus (TMV), a rod-like RNA virus, using X-ray crystallography. She first showed that, contrary to contemporary opinion, TMV virus particles were all of the same length. With Kenneth Holmes, she showed the virus's coat is composed of protein molecules arranged in helices. She designed and built a model of the virus to be exhibited at the 1958 World's Fair. She speculated that the virus is hollow, and correctly hypothesized that the RNA of TMV is single-stranded. Her work, together with that of Donald Caspar, revealed that the viral RNA is wound along the inner surface of the hollow virus. Her laboratory, which also included Aaron Klug, studied other plant viruses, including turnip yellow mosaic virus and viruses infecting potato, tomato and pea. Franklin also worked on icosahedral animal viruses, including poliovirus.
Franklin is commemorated in the Rosalind Franklin University of Medicine and Science.
In this month
May 1955: First issue of Virology; first English-language journal dedicated to virology
4 May 1984: HTLV-III, later HIV, identified as the cause of AIDS by Robert Gallo and coworkers
5 May 1939: First electron micrographs of tobacco mosaic virus taken by Helmut Ruska and coworkers
5 May 1983: Structure of influenza neuraminidase solved by Jose Varghese, Graeme Laver and Peter Colman
8 May 1980: WHO announced formally the global eradication of smallpox
11 May 1978: SV40 sequenced by Walter Fiers and coworkers
12 May 1972: Gene for bacteriophage MS2 coat protein is sequenced by Walter Fiers and coworkers, the first gene to be completely sequenced
13 May 2011: Boceprevir approved for the treatment of chronic hepatitis C virus (HCV) infection, the first direct-acting antiviral for HCV
14 May 1796: Edward Jenner inoculated James Phipps (pictured) with cowpox
15/16 May 1969: Death of Robert Rayford, the earliest confirmed case of AIDS outside Africa
18 May 1998: First World AIDS Vaccine Day
20 May 1983: Isolation of the retrovirus LAV, later HIV, by Luc Montagnier, Françoise Barré-Sinoussi and coworkers
23 May 2011: Telaprevir approved for the treatment of chronic HCV infection
25 May 2011: WHO declared rinderpest eradicated
31 May 1937: First results in humans from the 17D vaccine for yellow fever published by Max Theiler and Hugh H. Smith
Selected intervention
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The first Ebola vaccine was approved in 2019. Developed by the Public Health Agency of Canada, rVSV-ZEBOV is based on an attenuated recombinant vesicular stomatitis virus, genetically modified to express a surface glycoprotein of Zaire ebolavirus, and is estimated to be 97.5% effective. In the Kivu Ebola epidemic of 2018–20, a ring vaccination strategy was employed to protect direct and indirect contacts of infected people, as well as health workers, and around 300,000 people were vaccinated with rVSV-ZEBOV. A second vaccine was approved in 2020; this uses two different doses – a vector based on human adenovirus serotype 26 used to prime, boosted around eight weeks later by modified vaccinia Ankara (based on a heavily attenuated vaccinia virus) – and is not suitable for response to an outbreak. The efficacy is unknown. Multiple other vaccine candidates are in development to prevent Ebola, including replication-deficient adenovirus vectors, replication-competent human parainfluenza 3 vectors, and virus-like nanoparticle preparations.
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