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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

Hand washing is a protective measure against gastroenteritis

Gastroenteritis is an infectious disease of the gastrointestinal tract involving both the stomach and small intestine, which results in diarrhoea and vomiting, and sometimes abdominal pain. It can be caused by several types of virus: most commonly rotavirus and norovirus, but also adenovirus and astrovirus. Other major causes include Campylobacter, Escherichia coli, Vibrio cholerae and some other bacteria, as well as parasites. Viruses, particularly rotavirus, cause about 70% of gastroenteritis episodes in children, while norovirus is the leading cause of gastroenteritis among adults in America, causing over 90% of outbreaks.

Transmission can be from consumption of improperly prepared foods or contaminated water, or by close contact with infectious individuals. Good sanitation practices and a convenient supply of uncontaminated water are important for reducing infection. Personal measures such as hand washing can decrease incidence by as much as 30%. An estimated 3–5 billion cases of gastroenteritis occur globally each year, mainly among children and people in developing countries, resulting in 1.4 million deaths. Gastroenteritis is usually an acute and self-limiting disease that does not require medication; the main treatment is rehydration using oral rehydration therapy. A rotavirus vaccine is available.

Selected picture

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)

Selected article

Ribbon diagram of HIV reverse transcriptase

Reverse transcriptase is an enzyme that makes complementary DNA from an RNA template. It was discovered in 1970 by Howard Temin in Rous sarcoma virus, a retrovirus that causes tumours in chickens, and independently the same year by David Baltimore, for which the two shared the Nobel Prize in Physiology or Medicine. The discovery was initially controversial, as reverse transcription contradicts the central dogma of molecular biology, that information flows from DNA to RNA to protein.

Reverse transcription is essential for the replication of retroviruses, and allows them to integrate into the host genome as a provirus. The enzyme is a target for reverse-transcriptase inhibitors, an important class of anti-HIV drugs. The process is also important in eukaryotic genomes, in the replication of chromosome ends and retrotransposons, a type of mobile genetic element. Reverse transcriptase is widely used in the laboratory for molecular cloning, RNA sequencing, polymerase chain reaction and genome analysis.

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

Michael Kirby on the cost of antiviral drugs

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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

Diagram of HIV structure

Human immunodeficiency virus (HIV) is a lentivirus, an RNA virus in the retrovirus family. Two types of HIV have been characterised: HIV-1 is the more virulent and is responsible for most infections worldwide; HIV-2 is mainly confined to West Africa. The genome consists of two copies of a single-stranded +RNA, which contains nine genes. The roughly spherical virus particle has a diameter of about 120 nm; it is enveloped and contains a conical capsid made of around 2,000 copies of the p24 protein. The envelope glycoprotein, a trimeric complex of gp120 and gp41, binds to CD4, the primary receptor on the host cell.

HIV infects key cells in the human immune system including CD4+ helper T cells, macrophages and dendritic cells. Infection leads to low levels of CD4+ T cells via several mechanisms, resulting in a progressive immunodeficiency disease known as AIDS. Transmission occurs by the transfer of bodily fluids including blood, semen and breast milk, in which the virus is present both as free virus particles and within infected immune cells.

Did you know?

Clara Maass

Selected biography

Randy Shilts (8 August 1951 – 17 February 1994) was an American journalist, author and AIDS activist. The first openly gay reporter for a mainstream US newspaper, Shilts covered the unfolding story of AIDS and its medical, social, and political ramifications from the first reports of the disease in 1981. New York University's journalism department later ranked his 1981–85 AIDS reporting in the top fifty works of American journalism of the 20th century. His extensively researched account of the early days of the epidemic in the US, And the Band Played On: Politics, People, and the AIDS Epidemic, first published in 1987, brought him national fame. The book won the Stonewall Book Award and was made into an award-winning film. Shilts saw himself as a literary journalist in the tradition of Truman Capote and Norman Mailer. His writing has a powerful narrative drive, and interweaves personal stories with political and social reporting.

He received the 1988 Outstanding Author award from the American Society of Journalists and Authors, the 1990 Mather Lectureship at Harvard University, and the 1993 Lifetime Achievement Award from the National Lesbian and Gay Journalists' Association. He died of AIDS in 1994.

In this month

Dmitri Ivanovsky

February 1939: First virology journal, Archiv für die gesamte Virusforschung, appeared

8 February 1951: Establishment of the HeLa cell line from a cervical carcinoma biopsy, the first immortal human cell line

12 February 1892: Dmitri Ivanovsky (pictured) demonstrated transmission of tobacco mosaic disease by extracts filtered through Chamberland filters; sometimes considered the beginning of virology

19 February 1966: Prion disease kuru shown to be transmissible

27 February 2005: H1N1 influenza strain resistant to oseltamivir reported in a human patient

24 February 1977: Phi X 174 sequenced by Fred Sanger and coworkers, the first virus and the first DNA genome to be sequenced

28 February 1998: Publication of Andrew Wakefield's Lancet paper, subsequently discredited, linking the MMR vaccine with autism, which started the MMR vaccine controversy

Selected intervention

The MMR vaccine controversy centered around the – now discredited – notion that the combined vaccine for measles, mumps and rubella (MMR) might be associated with colitis and autism spectrum disorders. The idea was based on a research paper by Andrew Wakefield and co-authors, published in the prestigious medical journal The Lancet in 1998, and subsequently shown to be fraudulent. Sunday Times journalist Brian Deer's investigations revealed that Wakefield had manipulated evidence and had multiple undeclared conflicts of interest. The Lancet paper was retracted in 2010; Wakefield was found guilty of serious professional misconduct by the General Medical Council, and struck off the UK's Medical Register. The claims in Wakefield's article were widely reported in the press, resulting in a sharp drop in vaccination uptake in the UK and Ireland. A significantly increased incidence of measles and mumps followed, leading to deaths and severe injuries. Multiple large epidemiological studies have found no link between the vaccine and autism.

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