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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 microscope image of the cervix, showing normal epithelium (right) and carcinoma in situ (left), a pre-cancerous precursor to cervical cancer

Cervical cancer is a tumour of the cervix, the junction between the uterus and vagina in the female reproductive tract. Certain types of human papillomavirus (HPV) are implicated in more than 90% of these cancers, although the great majority of HPV infections of the cervix are not associated with cancer. HPV is transmitted by vaginal sex, infecting cervical epithelial cells. In 5–10% of cases, infection persists for years, and pre-cancerous changes called cervical intraepithelial neoplasia can develop. These can regress, but sometimes progress to cancer. Although in nearly all forms of the cancer, HPV infection is considered essential for cancer to develop, other risk factors are involved, including smoking, HIV infection and other forms of immune suppression.

Cervical cancer is the fourth most common cancer in women worldwide. It can be detected by screening; screening every 3–5 years, with appropriate follow-up, can reduce cancer incidence by up to 80%. HPV vaccines protect against types 16 and 18, which cause three-quarters of cancers. Where screening and vaccination are not available, cervical cancer has substantial mortality; worldwide, an estimated 528,000 cases and 266,000 deaths occurred in 2012, with 80% of these being in developing countries.

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Scanning electron micrograph of HIV budding from lymphocytes

HIV-1 budding from lymphocytes in culture. HIV establishes a latent infection in several types of immune cell and causes profound immunodeficiency.

Credit: C. Goldsmith (1984)

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Prion protein in its properly folded form

A prion is an infectious agent believed to be composed entirely of protein. This is in contrast to viruses and other known infectious agents, which all contain one or both of the nucleic acids, DNA and RNA. Prions propagate by transmitting a misfolded protein state. The prion induces existing, properly folded proteins in the host to convert into the misfolded prion form. This triggers a chain reaction resulting in large amounts of the prion form, disrupting cell function and causing cell death.

Prions are responsible for the transmissible spongiform encephalopathies in mammals. Human prion diseases include Creutzfeldt–Jakob disease, Gerstmann–Sträussler–Scheinker syndrome, fatal familial insomnia and kuru. Prion diseases of other mammals include bovine spongiform encephalopathy ("mad cow disease") in cattle and scrapie in sheep. All known mammalian prion diseases affect the structure of the brain or other neural tissue. All are currently untreatable and universally fatal. Proteins showing prion-type behaviour are also found in some fungi. Fungal prions do not appear to cause disease in their hosts.

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

Passengers in Mexico City wearing face masks in an attempt to prevent infection

The 2009 flu pandemic was an influenza pandemic first recognised in Mexico City in March 2009 and declared over in August 2010. It involved a novel strain of H1N1 influenza virus with genes from five different viruses, which resulted when a previous triple reassortment of avian, swine and human influenza viruses further combined with Eurasian swine influenza viruses, leading to the term "swine flu" being used for the pandemic. It was the second pandemic to involve an H1N1 strain, the first being the 1918 "Spanish flu" pandemic.

The global infection rate was 11–21%. This pandemic strain was less lethal than previous ones, killing about 0.01–0.03% of those infected, compared with 2–3% for Spanish flu. Estimates of global fatalities range from 284,500 to 579,000, mainly in Africa and Southeast Asia – not much above the normal seasonal influenza fatalities of 250,000–500,000 – leading to claims that the World Health Organization had exaggerated the danger.

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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


Rotavirus is a genus of double-stranded RNA viruses in the family Reoviridae. There are five species A–E; rotavirus A, the most common, causes over 90% of infections in humans. Rotavirus also infects animals, including livestock. The virus is transmitted by the faecal–oral route, with fewer than 100 virus particles being required for infection. Rotaviruses are stable in the environment and normal sanitary measures fail to protect against them. Effective rotavirus vaccines are the main prevention method.

The virus infects and damages the enterocytes lining the small intestine, causing gastroenteritis (sometimes referred to as "stomach flu," although the virus is not related to influenza). A viral toxin is responsible for some of the pathology. Rotavirus is the most common cause of severe diarrhoea among infants and young children. Almost every child worldwide has been infected with rotavirus at least once by the age of five. Over 500,000 children under five die from rotavirus infection each year and almost two million more become severely ill. Immunity develops with repeated infections and adults are rarely affected.

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Urera baccifera

Selected biography

Frederick Sanger

Frederick Sanger (13 August 1918 – 19 November 2013) was a British biochemist, the only person to have been awarded the Nobel Prize for Chemistry twice.

He started to research nucleic acid sequencing in the early 1960s. In 1975, he co-invented the "Plus and Minus" technique for sequencing DNA, which could sequence 80 nucleotides at once, a significant improvement on earlier techniques. Using this method, his group sequenced most of the 5,386 nucleotides of φX174 bacteriophage – the first virus and the first DNA genome to be completely sequenced – and showed that some of its genes overlapped. In 1977, he and his group pioneered the Sanger (or dideoxy chain-termination) method for sequencing DNA and used it to sequence the 48,502 bp bacteriophage λ. His technique remained the most widely used sequencing method until the mid-2000s, and was used to generate the first human genome sequence. Sanger is also known for sequencing bovine insulin, the first protein to be sequenced. The Sanger Institute was named for him.

In this month

Smallpox vaccination kit, including the bifurcated needle used to administer the vaccine

1 January 1934: Discovery of mumps virus by Claud Johnson and Ernest Goodpasture

1 January 1942: Publication of George Hirst's paper on the haemagglutination assay

1 January 1967: Start of WHO intensified eradication campaign for smallpox (vaccination kit pictured)

3 January 1938: Foundation of March of Dimes, to raise money for polio

6 January 2011: Andrew Wakefield's paper linking the MMR vaccine with autism described as "fraudulent" by the BMJ

25 January 1988: Foundation of the International AIDS Society

29 January 1981: Influenza haemagglutinin structure published by Ian Wilson, John Skehel and Don Wiley, the first viral membrane protein whose structure was solved

Selected intervention

Ball-and-stick model of aciclovir

Aciclovir (also acyclovir, Zovirax) is a nucleoside analogue, which mimics the nucleoside guanosine. After phosphorylation by viral thymidine kinase and cellular enzymes, it 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 Schaffer and colleagues, and developed by Schaffer 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. One of the most commonly used antiviral drugs, aciclovir is active against most viruses in the herpesvirus family. It is mainly used to treat herpes simplex virus infections, chickenpox and shingles. Aciclovir resistance is rare.

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