Portal:Viruses

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The Viruses Portal

The capsid of SV40, an icosahedral virus

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 5,000 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

Light micrograph of brain tissue showing the spongiform histological appearance of CJD

Creutzfeldt–Jakob disease, or CJD, is a rare fatal neurodegenerative disease, characterised by rapidly progressive dementia, and speech and movement impairment. The most common human transmissible spongiform encephalopathy, CJD is caused by a prion. Abnormal prion proteins build up as amyloid deposits in the brain, causing neurons to die. Affected brain tissue takes on a characteristic spongy appearance, with many round vacuoles in the grey matter.

The classic form of CJD can be caused by inheriting specific mutations in the PRNP gene (8% of cases) or by contact with infected human tissue via cadaver-derived hormone therapy, corneal or dura mater transplants, or contaminated surgical instruments (5%), but the majority (87%) of cases have no known cause. The first case of variant CJD, acquired by consuming food contaminated with the bovine spongiform encephalopathy (BSE) prion, or rarely via blood transfusion, occurred in 1994. Over 200 cases of symptomatic vCJD have since been recorded, mainly in the UK, and 1 in 2000 people in the UK have been estimated to be infected. No treatment is available. Prevention involves the control of BSE in cattle, restrictions on blood donors, replacement of cadaver-derived products, and heat and chemical decontamination of nondisposable surgical instruments.

Selected picture

Tobacco mosaic virus structure

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)

Selected article

Baltimore classification

Virus classification is the process of naming viruses and placing them into a taxonomic system. Viruses do not fit neatly into the biological classification system used for cellular organisms. They are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms and the type of disease they cause.

Two schemes are in common use. The Baltimore classification (pictured), proposed in 1971 by David Baltimore, places viruses into seven groups (I–VII) based on their nucleic acid type, number of strands and sense, as well as the method the virus uses to generate mRNA. The International Committee on Taxonomy of Viruses, established in the early 1970s, classifies viruses into orders, families, subfamilies, genera and species, similar to the groupings used for cellular organisms. As of 2012, seven orders had been defined, each containing viruses thought to have a common ancestor. The majority of the 96 defined families have not yet been assigned to an order.

In the news

Cryo-electron micrograph of Zika virus

18 January: The Coalition for Epidemic Preparedness Innovations commits $460 million to fast-tracking the development of vaccines against MERS coronavirus, Lassa virus and Nipah virus. BBC

18 November: WHO declares that Zika virus transmission and associated conditions (virus pictured) are a long-term situation which no longer qualifies as a Public Health Emergency of International Concern. WHO

4 October: Capsid protein assemblies of beak and feather disease virus, which infects endangered parrot species (infected bird pictured), are visualised by X-ray crystallography. Nat Commun

Sulphur-crested cockatoo with beak and feather disease

29 September: Zika virus is shown to infect the neural crest cells that develop into the cranium, driving cell death of neural progenitor cells by their impaired cytokine signalling. Cell Host Microbe

28 September: Ranaviruses (pictured), which cause severe disease in wild amphibians, are found to be spread in the UK by human activities. Proc R Soc London B

27 September: Rift Valley fever virus infection during pregnancy substantially increased the risk of miscarriage in a cross-sectional study in Sudan. Lancet Global Health

27 September: WHO declares measles to have been eliminated from North and South America. PAHO/WHO

26 September: Broadly neutralising antibodies to HIV are found in 239 HIV+ participants in the Swiss Cohort Study, with a significantly higher rate among black people. Nat Med

Transmission electron micrograph of ranaviruses infecting a cell

15 September: In a novel mouse model of hepatitis A virus infection, acute liver inflammation is found to be caused by hepatocyte apoptosis as an intrinsic response to infection. Science

13 September: A broadly neutralising antibody, which binds to the haemagglutinin stem and recognises most influenza A strains, is shown to be produced by human memory B cells. Nat Commun

2 September: Modelling suggests that the recently approved Sanofi-Pasteur vaccine may increase the frequency of severe dengue symptoms where viral transmission is low. Science

Selected outbreak

Notice prohibiting access to the North Yorkshire moors during the 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 England, southern Scotland and Wales, with Cumbria being the worst-affected area. A small outbreak occurred in the Netherlands, and there were a handful of cases elsewhere in Europe.

The UK outbreak was controlled by the beginning of October. Control measures included stopping animal movement and slaughtering over 10 million cows and sheep. Access to farmland and moorland was also restricted, 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 £8bn in the UK.

Selected quotation

Donald McNeil on the campaign to eradicate polio

Recommended articles

Viruses & Subviral agents: elephant endotheliotropic herpesvirus • HIV • introduction to virusesFeatured article • Playa de Oro virus • poliovirus • prion • rotavirusFeatured article • virusesFeatured article

Diseases: colony collapse disorder • common cold • croup • dengue feverFeatured article • gastroenteritis • Guillain–Barré syndrome • hepatitis B • hepatitis C • herpes simplex • HIV/AIDS • influenzaFeatured article • meningitisFeatured article • poliomyelitisFeatured article • shingles • smallpox

Epidemiology & Interventions: 1918 flu pandemic • 2007 Bernard Matthews H5N1 outbreak • 2009 flu pandemic • HIV/AIDS in Malawi • polio vaccine • West African Ebola virus epidemic

Host response: antibody • immune systemFeatured article • RNA interferenceFeatured article

Social & Media: And the Band Played On • Contagion • "Flu Season" • Frank's CockFeatured article • Race Against TimeFeatured article • social history of virusesFeatured article • "Steve Burdick" • "The Time Is Now"

People: Brownie Mary • Frank Macfarlane BurnetFeatured article • Aniru Conteh • HIV-positive peopleFeatured article • people with hepatitis CFeatured article • poliomyelitis survivorsFeatured article • Ryan WhiteFeatured article

Selected virus

False-coloured electron micrograph of Hendra virus

Henipaviruses are a genus of RNA viruses in the Paramyxoviridae family. The variably shaped, 40–600 nm diameter, enveloped capsid contains a single-stranded, negative-sense RNA genome of 18.2 kb, encoding six proteins. The cellular receptor Ephrin-B2 is widely distributed in mammals. Their natural hosts are bats, mainly megabats (fruit bats or flying foxes) and some microbats. Bats infected with Hendra virus develop viraemia and shed virus in urine, faeces and saliva for around a week, but show no signs of disease. Henipaviruses can also infect humans and livestock, causing severe disease with high mortality, making the group a zoonootic disease.

The first henipavirus, Hendra virus, was discovered in 1994 as the cause of an outbreak in horses in Brisbane, Australia. Two other species are known, Nipah and Cedar viruses. Their emergence as human pathogens has been linked to increased contact between bats and humans, sometimes via an intermediate domestic animal host. Human disease has been confined to Australia and Asia, but related sequences have also been found in African bats. Only a veterinary vaccine against Hendra virus is available.

Did you know?

A still life of flowers in a vase including striped tulips

Selected biography

Thomas Flewett in 1984

Thomas Flewett (29 June 1922 – 12 December 2006) was a British–Irish virologist and an authority on electron microscopy of viruses, best known for his role in the discovery of rotaviruses. After Ruth Bishop and others discovered viruses associated with diarrhoea, Flewett showed that they could be visualised by electron microscopy directly in faeces. He dubbed them "rotaviruses" for their wheel-shaped appearance. His group described the different rotavirus serotypes, and did extensive research on the rotavirus varieties infecting many animals.

Flewett established one of the first English virus laboratories in Birmingham in 1956. In addition to his rotavirus work, he discovered the cause of hand, foot and mouth disease, identified two new species of adenovirus, and co-discovered human torovirus and picobirnaviruses. His other research included influenza, coxsackie A, coxsackie B and hepatitis B viruses.

In this month

Electron micrograph of SARS coronaviruses

7 November 1991: Magic Johnson announced his retirement from basketball because of his infection with HIV

14 November 1957: Kuru, the first human prion disease, described by Daniel Gajdusek and Vincent Zigas

16 November 2002: The first case of severe acute respiratory syndrome (virus pictured) recorded in Guangdong, China

17 November 1995: Lamivudine approved for treatment of HIV

22 November 2013: Simeprevir approved for treatment of chronic hepatitis C virus infection

23 November 1978: Structure of tomato bushy stunt virus solved by Stephen Harrison and colleagues, the first atomic-level structure of a virus

24 November 2007: Outbreak of new Ebola species, Bundibugyo virus

26 November 1898: Martinus Beijerinck coined the term contagium vivum fluidum to describe the agent causing tobacco mosaic disease

Selected intervention

Ball-and-stick model of oseltamivir

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. If taken within 48 hours of infection, oseltamivir reduces the duration of influenza symptoms by about a day. 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, but cross-resistance to zanamivir is rare.

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