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.
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The first immortal human cell line, HeLa cells were derived from a cervical cancer biopsy and carry human papillomavirus 18 DNA. The cells have been growing since 1951.
Credit: Gerry Shaw (8 March 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
Virus quantification is necessary for viral vaccine production, and is used to manage people infected with HIV, hepatitis B and C, and cytomegalovirus. A wide range of traditional and modern methods are used. Plaque assays (pictured) infect a monolayer of host cells with dilutions of the virus and count the number of holes or plaques, where cells have lysed and infected their neighbours. For viruses that do not lyse their host cell, plaques of cells showing cytopathic effects can be counted or viral proteins can be immunostained with fluorescent-labelled antibodies. These methods quantify infectious virus, while others, such as electron microscopy, return a higher concentration because they count all virus particles, whether or not they are viable. Other assays, such as the haemagglutinin assay, quantify viral proteins.
Often slow and labour intensive, traditional methods have been complemented by modern technologies that greatly reduce quantification time, including quantitative polymerase chain reaction, flow cytometry, enzyme-linked immunosorbent assays and tunable resistive pulse sensing.
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.
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| “ | A virus is not an individual organism in the ordinary sense of the term, but something which could almost be called a stream of biological patterns. | ” |
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 coccolithovirus (pictured, arrow), a giant double-stranded DNA virus, has 472 protein-coding genes, and was the largest known marine virus by genome until the 2013 discovery of Pandoravirus salinus?
- ...that Tony Minson developed a new way of disabling viruses for vaccines?
- ...that the Oscar Award-nominated film The Final Inch, a documentary about efforts to eradicate the poliovirus, is the first film project from Google's philanthropic division Google.org?
- ...that a June 5, 1981, report by Joel Weisman in MMWR about five gay men with an unusual illness is recognized as the start of the AIDS pandemic and "the first report on AIDS in the medical literature"?
- ...that elastomeric respirators are used not only to protect against COVID-19 and tear gas, but also as fashion items?
Selected biography
George Keble Hirst (2 March 1909 – 22 January 1994) was an American virologist who was among the first to study the molecular biology and genetics of animal viruses.
Hirst started to work on influenza virus in 1940, only a few years after it had been isolated. He soon discovered that the virus caused red blood cells to clump together. This phenomenon could be used to diagnose influenza, which had previously required growing the virus in ferrets. He invented the haemagglutination assay, a simple method for quantifying viruses, and later the haemagglutination inhibition assay, which measures virus-specific antibodies in serum. In 1942, he discovered the neuraminadase enzyme, showing for the first time that viruses could contain enzymes. Neuraminidase is the target of the neuraminidase inhibitor class of antiviral drugs, including oseltamivir and zanamivir. In 1962, he was also the first to propose the then-revolutionary idea that virus genomes can consist of discontinuous segments.
He co-founded Virology in 1955, the first English-language journal to focus on viruses, and directed the Public Health Research Institute in New York City for nearly 25 years (1956–81).
In this month
5 June 1981: First report of HIV/AIDS (symbol pictured) appeared in medical literature
6 June 1997: Gene silencing in plants shown to be a viral defence mechanism
7–13 June 1962: Donald Caspar and Aaron Klug proposed the quasi-equivalence principle of virus structure
7–13 June 1962: André Lwoff proposed a viral classification scheme based on nature of genome, type of symmetry and presence of envelope
7–13 June 1962: George Hirst proposed that the influenza virus genome is segmented
9 June 1981: The American Society for Virology was founded
13 June 2012: First case of Middle East respiratory syndrome coronavirus (MERS-CoV) occurred in Saudi Arabia
18 June 1981: A vaccine against foot-and-mouth disease was the first genetically engineered vaccine
21 June 1996: Nevirapine approved, first NNRTI for HIV/AIDS
26 June 1993: Clinical trial of hepatitis B virus drug fialuridine terminated; the drug caused several fatalities due to lactic acidosis
28 June 2011: FAO declared rinderpest eradicated
30 June 1985: Ryan White was denied re-admittance to his school after an AIDS diagnosis, in a case that changed public perceptions of the disease
Selected intervention
Aciclovir (also acyclovir and sold as Zovirax) is a nucleoside analogue that mimics the nucleoside guanosine. It is active against most viruses in the herpesvirus family, and is mainly used to treat herpes simplex virus infections, chickenpox and shingles. After phosphorylation by viral thymidine kinase and cellular enzymes, the drug inhibits the viral DNA polymerase. Extremely selective and low in cytotoxicity, it was seen as the start of a new era in antiviral therapy. Aciclovir was discovered by Howard Schaeffer and colleagues, and developed by Schaeffer and Gertrude Elion, who was awarded the 1988 Nobel Prize in Medicine in part for its development. Nucleosides isolated from a Caribbean sponge, Cryptotethya crypta, formed the basis for its synthesis. Aciclovir differs from earlier nucleoside analogues in containing only a partial nucleoside structure: the sugar ring is replaced with an open chain. Resistance to the drug is rare in people with a normal immune system.
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