The Viruses Portal
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![The capsid of SV40, an icosahedral virus](http://upload.wikimedia.org/wikipedia/commons/thumb/6/6a/Symian_virus.png/220px-Symian_virus.png)
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
Ebola virus disease is a viral haemorrhagic fever of humans and other primates caused by ebolaviruses, RNA viruses in the Filoviridae family. The natural host is thought to be megabats. The virus spreads by contact with body fluids, such as blood. After an incubation period of 2–21 days, nonspecific symptoms including fever, sore throat, muscle pain and headache develop, followed by vomiting, diarrhoea, rash, liver and kidney impairment, and sometimes internal or external bleeding. On average, around half of those infected die, often from hypovolaemic shock. The virus sometimes persists in semen or breast milk for several weeks or months after recovery.
The disease was first identified in 1976 by Peter Piot and others in two simultaneous outbreaks, one in Nzara, Sudan and the other in Yambuku, then in Zaire, near the Ebola River, for which the disease is named. Outbreaks have occurred intermittently in tropical regions of sub-Saharan Africa, with a few thousand cases documented before the epidemic in Western Africa of 2013–16, in which nearly 29,000 cases with more than 11,000 deaths were reported. More than 3000 cases were documented in a subsequent epidemic in the Democratic Republic of the Congo in 2018–20, with more than 2000 deaths. Prevention of outbreaks focuses on limiting virus transmission from infected animals to humans, for example, by taking precautions when handling bush meat. Ebola vaccines are now available. Monoclonal antibody treatments can improve outcomes, and a combination of three monoclonals has been licensed.
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
Interferons are a group of signalling proteins released by host cells in response to viruses and other pathogens. They belong to a large class of proteins known as cytokines: molecules used for communication between cells to trigger the protective defences of the immune system that help to eradicate pathogens. More than twenty distinct interferon genes and proteins have been identified in animals, including humans.
When a cell is infected by a virus, several virus products, including glycoproteins and viral RNA, stimulate the cell to produce and release interferons. This causes nearby cells to heighten their defences against viral infection, and so interferes with viral replication. Some viruses, including Japanese encephalitis virus, dengue type 2 virus, human cytomegalovirus and Kaposi's sarcoma-associated herpesvirus, have evolved ways to resist interferon's antiviral activity. Interferons also have various other functions in regulating the immune system. Interferons and other cytokines are responsible for some symptoms of infection, such as fever, muscle pain and "flu-like symptoms".
Selected outbreak
The West African Ebola epidemic was the most widespread outbreak of the disease to date. Beginning in Meliandou in southern Guinea in December 2013, it spread to adjacent Liberia and Sierra Leone, affecting the cities of Conakry and Monrovia, with minor outbreaks in Mali and Nigeria. Cases reached a peak in October 2014 and the epidemic was under control by late 2015, although occasional cases continued to occur into April 2016. Ring vaccination with the then-experimental vaccine rVSV-ZEBOV was trialled in Guinea.
More than 28,000 suspected cases were reported with more than 11,000 deaths; the case fatality rate was around 40% overall and around 58% in hospitalised patients. Early in the epidemic nearly 10% of the dead were healthcare workers. The outbreak left about 17,000 survivors, many of whom reported long-lasting post-recovery symptoms. Extreme poverty, dysfunctional healthcare systems, distrust of government after years of armed conflict, local burial customs of washing the body, the unprecedented spread of Ebola to densely populated cities, and the delay in response of several months all contributed to the failure to control the epidemic.
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
West Nile virus (WNV) is a flavivirus, an RNA virus in the Flaviviridae family. The enveloped virion is 45–50 nm in diameter and contains a single-stranded, positive-sense RNA genome of around 11,000 nucleotides, encoding ten proteins. The main natural hosts are birds (the reservoir) and several species of Culex mosquito (the vector). WNV can also infect humans and some other mammals, including horses, dogs and cats, as well as some reptiles. Transmission to humans is generally by bite of the female mosquito. Mammals form a dead end for the virus, as it cannot replicate sufficiently efficiently in them to complete the cycle back to the mosquito.
First identified in Uganda in 1937, WNV was at first mainly associated with disease in horses, with only sporadic cases of human disease until the 1990s. The virus is now endemic in Africa, west and central Asia, Oceania, the Middle East, Europe and North America. A fifth of humans infected experience West Nile fever, a flu-like disease. In less than 1% of those infected, the virus invades the central nervous system, causing encephalitis, meningitis or flaccid paralysis. No specific antiviral treatment has been licensed and only a veterinary vaccine is available. Mosquito control is the main preventive measure.
Did you know?
- ...that Franquet's Epauletted Fruit Bat (pictured) is one of three fruit-eating bats found to be a reservoir for Ebola virus in the wild?
- ...that 95% of adults have been infected by human herpesvirus 7, a virus that can cause influenza-like illness and seizures but normally causes no symptoms?
- ...that James Jurin used statistical studies to show the probability of death from smallpox vaccine was significantly less than from smallpox?
- ...that the log-Cauchy distribution has been proposed as a model for the progression of HIV in individuals?
- ...that Corona, named after Corona, is fighting corona?
Selected biography
Edward Jenner (1749–1823) was an English physician and scientist who pioneered the smallpox vaccine, the world's first vaccine. Noting the common observation that milkmaids were generally immune to smallpox, Jenner postulated that the pus in the blisters that milkmaids received from cowpox (a similar but much less virulent disease) protected them from smallpox. In 1796, Jenner tested his hypothesis by inoculating an eight-year-old boy with pus from an infected milkmaid. He subsequently repeatedly challenged the boy with variolous material, then the standard method of immunisation, without inducing disease. He published a paper including 23 cases in 1798. Although several others had previously inoculated subjects with cowpox, Jenner was the first to show that the procedure induced immunity to smallpox. He later successfully popularised cowpox vaccination.
Jenner is often called "the father of immunology", and his work is said to have saved more lives than that of any other individual.
In this month
1 July 1796: Edward Jenner first challenged James Phipps with variolation, showing that cowpox inoculation is protective against smallpox
3 July 1980: Structure of southern bean mosaic virus solved by Michael Rossmann and colleagues
6 July 1885: Louis Pasteur (pictured) gave rabies vaccine to Joseph Meister
10 July 1797: Jenner submitted paper on Phipps and other cases to the Royal Society; it was read to the society but not published
14–20 July 1968: First International Congress for Virology held in Helsinki
16 July 2012: FDA approved tenofovir/emtricitabine (Truvada) for prophylactic use against HIV; first prophylactic antiretroviral
19 July 2013: Pandoravirus described, with a genome twice as large as Megavirus
22 July 1966: International Committee on Nomenclature of Viruses (later the ICTV) founded
25 July 1985: Film star Rock Hudson made his AIDS diagnosis public, increasing public awareness of the disease
28 July 2010: First global World Hepatitis Day
Selected intervention
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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