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
Rabies is a disease of humans and some other mammals, generally caused by the rabies virus, an RNA virus in the Rhabdoviridae family. A few cases have involved the closely related Australian bat lyssavirus. Rabies virus has a wide host range. Globally, dogs are the main source of human infections, with bats being important in the Americas; other naturally infected animals include monkeys, raccoons, foxes, skunks, cattle, horses, wolves, coyotes, cats, mongooses, bears, groundhogs, weasels and other carnivores. Transmission is commonly via saliva, usually but not always from bites; it can potentially occur via aerosols contacting mucous membranes. The typical human incubation period is 1–3 months. The neurotropic virus travels along neural pathways into the CNS and brain, where it causes meningoencephalitis. Nonspecific symptoms such as fever and headache are followed by neurological symptoms, including partial paralysis, confusion, agitation, paranoia, hallucinations and sometimes hydrophobia, which progress to delirium, coma and death. Around 17,400 people died from rabies in 2015, mainly in Asia and Africa.
Rabies is mentioned in the Codex of Eshnunna of around 1930 BC. The first vaccine was developed in 1885 by Louis Pasteur and Émile Roux. Prophylactic vaccination is used in people at high risk, pets and wild animals. Post-exposure prophylaxis, including vaccine and immunoglobulin, is completely effective if begun immediately after exposure, but survival is extremely rare once symptoms have begun.
Selected image
Tobacco mosaic virus was the first virus to be identified, as an infectious agent that could pass through porcelain filters, as well as the first to be crystallised. It was among the earliest virus structures to be modelled successfully.
Credit: Thomas Splettstoesser (20 July 2012)
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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The Hershey–Chase experiments were conducted by Alfred Hershey and Martha Chase in 1952 using the T2 bacteriophage (pictured), which is composed of DNA wrapped in a protein shell. Hershey and Chase labelled either the phage DNA using radioactive phosphorus-32 or the protein using radioactive sulphur-35. They allowed the radiolabelled phages to infect unlabelled bacteria, and then agitated in a blender and centrifuged to separate material remaining outside the bacterial cells. The majority of the 32P-labelled DNA entered the host bacterial cell, while all the 35S-labelled protein remained outside. Hershey and Chase also showed that the phage DNA is inserted into the bacteria shortly after the virus attaches to its host.
DNA had been known since 1869, but in 1952 many scientists believed that proteins carried the information for inheritance. Proteins appeared more complex, while DNA was thought to be an inert molecule used for phosphorus storage. These experiments built on earlier research on transformation in bacteria and helped to confirm that DNA, not protein, was the genetic material. Hershey shared the 1969 Nobel Prize in Physiology or Medicine for the research.
Selected outbreak
The 1918–20 influenza pandemic, the first of the two involving H1N1 influenza virus, was unusually deadly. It infected 500 million people across the entire globe, with a death toll of 50–100 million (3–5% of the world's population), making it one of the deadliest natural disasters of human history. It has also been implicated in the outbreak of encephalitis lethargica in the 1920s. Despite the nickname "Spanish flu", the pandemic's geographic origin is unknown.
Most influenza outbreaks disproportionately kill young, elderly or already weakened patients; in contrast this predominantly killed healthy young adults. Contemporary medical reports suggest that malnourishment, overcrowded medical facilities and poor hygiene promoted fatal bacterial pneumonia. Some research suggests that the virus might have killed through a cytokine storm, an overreaction of the body's immune system. This would mean the strong immune reactions of young adults resulted in a more severe disease than the weaker immune systems of children and older adults.
Selected quotation
| “ | Some scientists visualize the virus as an ill-defined shape emerging bashfully out of a dense and golden cloud. This is a beautiful and romantic vision. Virology should, however, not be too Turnerian. Nor should it be an abstract art. The portrait of a virus should not produce an aesthetic emotion by means of an organic disturbance. | ” |
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
Acanthamoeba polyphaga mimivirus (APMV) is a species of DNA virus in the Mimivirus genus of the Mimiviridae family. It infects the amoeba, Acanthamoeba polyphaga. Its non-enveloped icosahedral capsid is 400 nm in diameter, with protein filaments of 100 nm projecting from its surface. The APMV genome is a linear, double-stranded DNA molecule of around 1.2 megabases, encoding around 979 genes. This is comparable to the genome of some small bacteria. It encodes several proteins that had not been previously discovered in viruses, including aminoacyl tRNA synthetases. APMV is itself parasitised by the Sputnik virophage (arrowed in micrograph).
APMV is as large as some small species of bacteria, such as Rickettsia conorii and Tropheryma whipplei. When it was first discovered in 1992, it was thought to be a bacterium, and named Bradfordcoccus. APMV was not shown to be a virus until 2003, when it was the largest virus then discovered. It has since been overtaken by Megavirus chilensis, Pandoravirus and Pithovirus, all of which also infect amoebae. These and other large and complex DNA viruses are now grouped in Nucleocytoviricota, also termed nucleocytoplasmic large DNA viruses.
Did you know?
- ...that flow cytometry bioinformaticians use methods from computational statistics and machine learning to analyse single cell data gathered by flow cytometry for cancer and HIV/AIDS research (output pictured)?
- ...that Sierra Leonean physician Aniru Conteh saved thousands of lives from Lassa fever before dying from the disease himself?
- ...that early in the history of veterinary medicine in the Philippines, a cattle plague killed 600,000 animals from 1901 to 1902 alone?
- ...that the isolation of antibodies and flu viruses from birds on Tryon Island, a coral cay off the coast of Queensland, Australia, led to the development of antiviral drugs, such as oseltamivir?
- ...that the Plague of Athens devastated ancient Athens in 430 BC, perhaps leading ultimately to the city's defeat in the Peloponnesian War?
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 April 1911: Peyton Rous showed that a cell-free isolate could transmit sarcoma in chickens, an early demonstration of cancer caused by a virus
7 April 1931: First electron micrograph taken by Ernst Ruska and Max Knoll
8 April 1976: Bacteriophage MS2 (pictured) sequenced by Walter Fiers and coworkers, first viral genome to be completely sequenced
8 April 1990: Death from AIDS of Ryan White, haemophiliac teenager for whom the Ryan White Care Act is named
8 April 1992: Tennis player Arthur Ashe announced that he had been infected with HIV from blood transfusions
9 April 1982: Stanley Prusiner proposed proteinaceous prions as the cause of scrapie
12 April 1955: Success of trial of Jonas Salk's polio vaccine announced
12 April 2013: New order of double-stranded DNA bacteriophages, Ligamenvirales, announced
15 April 1957: André Lwoff proposes a concise definition of a virus
21 April 1989: Discovery of hepatitis C virus by Qui-Lim Choo and colleagues
28 April 1932: First yellow fever vaccine announced at an American Societies for Experimental Biology meeting by Wilbur Sawyer
29 April 2015: PAHO and WHO declared the Americas region free from rubella transmission
30 April 1937: Discovery of Theiler's murine encephalomyelitis virus, later a model for multiple sclerosis research
Selected intervention
Oseltamivir (also Tamiflu) is an oral antiviral drug against influenza (flu). It was the second inhibitor of the viral neuraminidase to be developed, after zanamivir, and the first to be taken as an oral tablet. It was originally synthesised from shikimic acid extracted from the star anise plant. Oseltamivir is a prodrug that requires metabolism in the liver to the active form, oseltamivir carboxylate. This binds at the active site of the neuraminidase enzyme, preventing it from cleaving sialic acid to release the virus particle from the host cell. Oseltamivir can reduce the duration of influenza symptoms by 0.5–1 days. Debate is ongoing about whether it also reduces the risk of complications, such as pneumonia. Nausea and vomiting are the main adverse events. Resistance to oseltamivir has been observed in some strains of influenza virus, especially H1N1 strains.
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