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
Smallpox is an eradicated infectious disease of humans caused by the Variola major and V. minor viruses. V. major causes a serious disease with a mortality rate of around 30%; V. minor is associated with much milder symptoms and mortality below 1%. The virus is mainly transmitted by the respiratory route but can also be carried on contaminated objects. Smallpox preferentially infects skin cells, resulting in a usually maculopapular rash, and later, raised fluid-filled blisters. Most V. major survivors have permanent scarring, commonly on the face, which can be extensive. Less common long-term complications include blindness resulting from corneal ulceration and scarring, and in young children, limb deformities due to arthritis and osteomyelitis.
Smallpox probably emerged in human populations in about 10,000 BC; the mummified body of Egyptian pharaoh Ramses V shows evidence of smallpox rash. The disease was responsible for an estimated 300–500 million deaths during the 20th century. Smallpox vaccine, the earliest vaccine, was developed in the 18th century. After intensive vaccination campaigns, the last natural infection occurred in 1977. Smallpox was certified the first infectious disease to be eradicated globally in 1979. Debate is ongoing over whether all stocks of the virus should be destroyed.
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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
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
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
Alphaviruses are a genus of RNA viruses in the Togaviridae family. The spherical enveloped virion is 70 nm in diameter, with a nucleocapsid of 40 nm. It has a single-stranded, positive-sense RNA genome of 11–12 kb. The genus contains more than thirty species, which infect humans, horses, rodents and other mammals, as well as fish, birds, other vertebrates and invertebrates. Alphaviruses are generally transmitted by insect vectors, predominantly mosquitoes, and are an example of arboviruses (arthropod-borne viruses).
The first alphavirus to be discovered was western equine encephalitis virus, by Karl Friedrich Meyer in 1930, in horses with fatal encephalitis in San Joaquin Valley, California, USA. Some members of the genus cause significant disease in humans, including the chikungunya, o'nyong'nyong, Ross River, Sindbis, Barmah Forest and Semliki Forest (pictured) viruses and the eastern, western and Venezuelan equine encephalitis viruses. Arthritis, encephalitis, rashes and fever are the most frequently observed symptoms. Large mammals such as humans usually form dead-end hosts for the viruses, although Venezuelan equine encephalitis virus is mainly amplified in the horse. No human vaccine or antiviral drug has been licensed. Prevention is predominantly by control of the insect vector.
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
Many well-known people have survived the paralytic disease polio. The earliest identified case might be Siptah (pictured), Egyptian pharaoh 1197–1191 BC, whose mummified remains have a deformed leg possibly from polio. Claudius, Roman emperor 41–54 AD, walked with a limp after a childhood disease that historians have hypothesised might have been polio. Novelist Sir Walter Scott suffered paralysis in one leg after a teething fever in 1773, which left him lame; his detailed account of his disease has allowed a retrospective diagnosis of polio to be made with confidence.
For many of those who survived it, paralytic polio was a life-changing experience. The disease can lead to permanent physical disability; Itzhak Perlman, for example, plays the violin seated. Others recover completely, with some going on to excel in sports; Ray Ewry became world's foremost standing jumper after childhood polio. Some survivors, including singer Ian Dury and actress Mia Farrow, have campaigned for polio eradication or for disability rights.
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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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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