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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

Cold sore on the lower lip (arrow)

Herpes simplex is caused by herpes simplex virus types 1 and 2 of the Herpesviridae family. The most common forms of infection are oral herpes, which results in cold sores, and genital herpes. Active disease usually involves blisters containing infectious virus, although the genital form is often asymptomatic. Rare disorders associated with herpes simplex include herpetic whitlow, herpes gladiatorum, ocular herpes, cerebral herpes infection encephalitis, Mollaret's meningitis, neonatal herpes and possibly Bell's palsy.

After initial infection, virus particles are transported along sensory nerves to the sensory nerve cell bodies, where they become latent and remain lifelong. Periods of remission alternate with outbreaks of active disease lasting 2–21 days, in which the virus multiplies in the nerve cell and new virus particles are transported along the neuronal axon to the nerve terminals in the skin, where they are released. What causes these recurrences is unclear, although immunosuppressants are known to be one trigger. Transmission is usually by direct contact with a lesion or with body fluids, and can occur during periods of asymptomatic shedding. Barrier protection methods reduce the risk. No cure exists, but antiviral treatment can alleviate symptoms and reduce viral shedding.

Selected picture

Portrait of Louis Pasteur by Albert Edelfelt (1885)

Louis Pasteur invented a vaccine against rabies, and tested it on a boy bitten by a rabid dog in 1885.

Credit: Albert Edelfelt (1885)

Selected article

Ribbon diagram of the Dicer enzyme from Giardia intestinalis

RNA interference is a type of gene silencing that forms an important part of the immune response against viruses. A cell enzyme called Dicer (pictured) cleaves double-stranded RNA molecules found in the cell cytoplasm – such as the genome of an RNA virus – into short fragments, which are separated into single strands and integrated into a large multi-protein complex. Here they recognise their complementary messenger RNA molecules and target them for destruction. This inhibits, or silences, the expression of viral genes.

RNA interference is a key part of the adaptive immune system in plants, allowing the entire plant to respond to a virus after a localised encounter. The protective effect can be transferred between plants by grafting. Many plant viruses have evolved elaborate mechanisms to suppress this response. The process is also important in innate immunity towards viruses in insects. Little is known about the role of RNA interference in mammals.

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

Map showing Ebola virus disease cases in Guinea, Liberia and Sierra Leone in December 2014

The West African Ebola epidemic was the most widespread outbreak of the disease to date, and the first to occur outside Sub-Saharan Africa. Beginning in Meliandou in southern Guinea in December 2013, it spread to adjacent Liberia and Sierra Leone, affecting the densely populated cities of Conakry and Monrovia, with minor outbreaks in Mali and Nigeria. The epidemic was under control by late 2015, but occasional cases continued to occur into June of the following year. More than 28,000 suspected cases were reported with more than 11,000 deaths, a case fatality rate of around 58% in hospitalised patients and up to 70% overall. Around 10% of the dead were healthcare workers.

Extreme poverty, a dysfunctional healthcare system, a distrust of government officials after years of armed conflict, local burial customs involving washing the body after death, and a delay in response of several months all contributed to the failure to contain the epidemic.

Selected quotation

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 Sputnik virophage

Sputnik virophage is a DNA virus, discovered in 2008, that infects Acanthamoeba protozoa. It is a satellite virus of the giant mamavirus. It requires mamavirus to infect the cell simultaneously to replicate, hijacking the virus factories that mamavirus 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 other satellite viruses that infect plants, arthropods and mammals is disputed. Other virophages have since been discovered, including the Mavirus, Zamilon and Organic Lake virophages; all infect protists and all rely on nucleocytoplasmic large DNA viruses as helpers. They have been proposed to belong to a new family, Lavidaviridae.

Sputnik's non-enveloped icosahedral capsid is 50 nm in diameter, and contains a circular double-stranded DNA genome of 18.3 kb. Three of its 21 predicted proteins are thought to derive from mamavirus or the related 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?

Atlantic salmon

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 zidovudine

Zidovudine (ZDV) (also known as AZT and Retrovir) is an antiretroviral drug used in the treatment of HIV/AIDS. Classed as a nucleoside analogue reverse-transcriptase inhibitor, it inhibits HIV's reverse transcriptase enzyme, which copies the viral RNA into DNA and is essential for its replication. The first breakthrough in AIDS therapy, ZDV was licensed in 1987. While it significantly reduces HIV replication, leading to clinical and immunological benefits, when used alone ZDV does not completely stop replication, allowing the virus to become resistant to it. The drug is therefore used together with other anti-HIV drugs in combination therapy called highly active antiretroviral therapy (HAART). To simplify its administration, ZDV is included in Combivir, Trizivir and other combination pills. ZDV can also be used to prevent HIV transmission, such as from mother to child during childbirth or after a needlestick injury.

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