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
Zika fever is caused by the Zika virus, a mosquito-borne flavivirus first isolated in Uganda in 1947. Human infection is usually asymptomatic; a minority of cases have mild symptoms, which generally last 2–7 days and can include fever, conjunctivitis, joint pain, headache and a maculopapular rash. Infections in adults have occasionally been associated with Guillain–Barré syndrome. The incubation period is probably up to a week. The natural reservoir is unknown. The virus is transmitted by Aedes mosquitoes, mainly Aedes aegypti in tropical regions. During pregnancy, mother-to-child transmission can cause microcephaly (pictured) and other brain malformations in some babies. In one study, major abnormalities were seen in 42% of live births. The virus is present in semen and male-to-female sexual transmission has been documented; it is also found in breast milk and blood.
The first documented human outbreak occurred in 2007 in the Federated States of Micronesia. The virus is thought to have entered the Americas in around 2013, and an outbreak started in Brazil in 2015, spreading across the Americas and to Pacific, Asia and Africa, and leading the World Health Organization to consider Zika a Public Health Emergency of International Concern in February–November 2016. No specific treatment nor vaccine has been approved. Prevention involves mosquito control and condom use; women in areas where Zika was circulating were recommended to consider delaying pregnancy, and pregnant women were advised to avoid travel to affected areas.
Selected image
Chikungunya virus is an alphavirus transmitted by Aedes mosquitoes. The disease can cause severe joint pain, sometimes lasting for several months. Outbreaks have occurred across Africa, Asia and India, and in 2013–14, in South America and the Caribbean.
Credit: A2-33 (8 December 2013)
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
The 1993 hantavirus outbreak in the Four Corners region of southwest USA was of a novel hantavirus, subsequently named Sin Nombre virus. It caused the previously unrecognised hantavirus pulmonary syndrome – the first time that a hantavirus had been associated with respiratory symptoms. Mild flu-like symptoms were followed by the sudden onset of pulmonary oedema, which was fatal in half of those affected. A total of 24 cases were reported in April–May 1993, with many of those affected being from the Navajo Nation territory. Hantavirus infection of humans generally occurs by inhaling aerosolised urine and faeces of rodents, in this case the deer mouse (Peromyscus; pictured).
Previously documented hantavirus disease had been confined to Asia and Europe, and these were the first human cases to be recognised in the USA. Subsequent investigation revealed undiagnosed cases dating back to 1959, and Navajo people recalled similar outbreaks in 1918, 1933 and 1934.
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
Noroviruses are a genus of non-enveloped, single-stranded RNA viruses in the family Caliciviridae. The positive-sense RNA genome is approximately 7500 nucleotides long. Known noroviruses fall into five different genogroups (GI–GV); three groups infect humans, the other two mice, and cattle and other bovines. All are considered strains of a single species, Norwalk virus.
Noroviruses are extremely contagious, with fewer than 20 virus particles being infectious. They are transmitted directly from person to person and indirectly via contaminated water and food. After infection, the virus replicates in the small intestine, causing acute gastroenteritis, which develops 12–48 hours after exposure and lasts for 24–72 hours. The characteristic symptoms include nausea, forceful vomiting, watery diarrhoea and abdominal pain. Infection is usually self-limiting and rarely severe. Noroviruses cause 18% of acute gastroenteritis episodes in humans, with around 685 million cases and 200,000 deaths every year, mainly in very young, elderly or immunosuppressed people. No vaccine is available. Hand washing with soap and water is effective in reducing transmission.
Did you know?
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- ...that infectious disease specialist Daniel R. Lucey has hypothesised that the SARS-CoV-2 virus (pictured) responsible for the 2019–20 coronavirus outbreak may have been quietly circulating among humans since at least November 2019?
- ...that the Kunjin virus, which can be transmitted by mosquitoes and may cause encephalitis in humans, is named for an Indigenous Australian clan living near where the virus was first isolated?
- ...that the Vaccine Confidence Project was developed in response to a boycott of polio eradication efforts in Nigeria?
- ...that the Gaussian network model has a wide range of applications from enzymes composed of a single domain, to large macromolecular assemblies, such as ribosomes and viral capsids?
- ...that Trinidadian virologist Joseph Lennox Pawan was the first person to show that rabies could be spread by vampire bats to other animals and humans?
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
6 October 2008: Nobel Prize in Physiology or Medicine awarded to Harald zur Hausen for showing that human papillomaviruses cause cervical cancer, and to Françoise Barré-Sinoussi and Luc Montagnier for discovering HIV
7 October 2005: 1918 Spanish influenza pandemic strain reconstituted
9 October 1991: Didanosine was the second drug approved for HIV/AIDS
12 October 1928: First use of an iron lung in a poliomyelitis patient
12 October 2007: Raltegravir (pictured) approved; first HIV integrase inhibitor
14 October 1977: Habiba Nur Ali was the last person to die from naturally occurring smallpox
14 October 2010: Rinderpest eradication efforts announced as stopping by the UN
16 October 1975: Last known case of naturally occurring Variola major smallpox reported
25 October 2012: Alipogene tiparvovec, a gene therapy for lipoprotein lipase deficiency using an adeno-associated virus-based vector, was the first gene therapy to be licensed
26 October 1977: Ali Maow Maalin developed smallpox rash; the last known case of naturally occurring Variola minor smallpox
26 October 1979: Smallpox eradication in the Horn of Africa formally declared by WHO, with informal declaration of global eradication
27 October 2015: Talimogene laherparepvec was the first oncolytic virus to be approved by the FDA to treat cancer
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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