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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

Child with a measles rash

Measles is a disease that only affects humans caused by the measles virus, an RNA virus in the Paramyxoviridae family. It is highly contagious, with transmission occurring via the respiratory route or by contact with secretions. Symptoms generally develop 10–12 days after exposure and last 7–10 days; they include high fever, cough, rhinitis and conjunctivitis, white Koplik's spots inside the mouth and a generalised red maculopapular rash. Complications occur in about 30% of those infected, and include diarrhoea, pneumonia, bronchitis, otitis media, encephalitis, corneal ulceration and blindness. The risk of death is usually around 0.1–0.3%, but may be as high as 10–28% in areas with high levels of malnutrition.

Measles was first described by Rhazes (860–932). The disease is estimated to have killed around 200 million people between 1855 and 2005. It affects about 20 million people a year, primarily in the developing areas of Africa and Asia; as of 2013, it causes the most vaccine-preventable deaths of any disease, at about 96,000 annually. No antiviral drug is licensed. An effective measles vaccine is available, but uptake has been reduced by anti-vaccination campaigns, particularly the fraudulent claim that the MMR vaccine might be associated with autism.

Selected picture

Aedes aegypti mosquito biting a human

Aedes aegypti can transmit the chikungunya, dengue, yellow fever and Zika viruses. The mosquito is widespread in tropical and subtropical regions, with mosquito control being key to disease prevention.

Credit: United States Department of Agriculture (2000)

Selected article

16th-century Aztec print showing a person with measles

Viruses have infected plants and animals, including humans, for millions of years. Epidemics caused by viruses began when human behaviour changed during the Neolithic period. Previously hunter-gatherers, humans developed more densely populated agricultural communities, which allowed viruses to spread rapidly and subsequently to become endemic. Louis Pasteur and Edward Jenner were the first to develop vaccines to protect against viral infections, long before viruses were discovered. The sizes and shapes of viruses remained unknown until the invention of the electron microscope in the 1930s, when the science of virology gained momentum. In the 20th century, many diseases were found to be caused by viruses.

Viruses are the most abundant biological entity on Earth. Although scientific interest in them arose because of the diseases they cause, most viruses are beneficial. They have driven evolution by transferring genes across species, play important roles in ecosystems, and are essential to life.

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

Quarantine notices at the East Birmingham Hospital where the first case was initially treated

The last recorded smallpox death occurred during the 1978 smallpox outbreak in Birmingham, UK. The outbreak resulted from accidental exposure to Variola major, probably the Abid strain, from a WHO-funded laboratory, headed by Henry Bedson, at the University of Birmingham Medical School – also the probable source of a 1966 outbreak. Bedson was investigating strains of smallpox known as whitepox, considered a potential threat to the smallpox eradication campaign, then in its final stages. The virus appears to have spread between floors in late July, possibly via ducting, to infect a medical photographer who worked above the laboratory. She showed symptoms in August and died the following month; one of her contacts was also infected but survived. Bedson committed suicide while under quarantine.

A government inquiry into the outbreak criticised the university's safety procedures. Radical changes in UK research practices for handling dangerous pathogens followed, and all known stocks of smallpox virus were concentrated in two laboratories.

Selected quotation

Donald McNeil on the campaign to eradicate polio

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Selected virus

Electron micrograph of Acanthamoeba polyphaga mimivirus

Acanthamoeba polyphaga mimivirus (APMV) is the only species in the Mimivirus genus of the Mimiviridae family of DNA viruses. It infects the amoeba, Acanthamoeba polyphaga. Its non-enveloped icosahedral capsid is 400 nm in diameter, with protein filaments of 100 nm projecting from its surface. The APMV genome is a linear, double-stranded DNA molecule of around 1.1 megabases, encoding around 979 genes. This is comparable to the genome of some small bacteria. It encodes several proteins that had not been previously discovered in viruses, including aminoacyl tRNA synthetases.

APMV is as large as some small species of bacteria. When it was first discovered in 1992, it was thought to be a bacterium, and named Bradfordcoccus. APMV was not shown to be a virus until 2003, when it was the largest virus then discovered. It has since been overtaken by Megavirus chilensis, Pandoravirus and Pithovirus, all of which also infect amoebae. These and other large viruses have been called "nucleocytoplasmic large DNA viruses", an informal grouping of very large and complex DNA viruses.

Did you know?

Two greenbugs (Schizaphis graminum)

Selected biography

Françoise Barré-Sinoussi in 2008

Françoise Barré-Sinoussi (born 30 July 1947) is a French virologist, known for being one of the researchers who discovered HIV.

Barré-Sinoussi researched retroviruses in Luc Montagnier's group at the Institut Pasteur in Paris. In 1982, she and her co-workers started to analyse samples from people with a new disease, then referred to as "gay-related immune deficiency". They found a novel retrovirus in lymph node tissue, which they called "lymphadenopathy-associated virus". Their results were published simultaneously with those of Robert Gallo's group in the USA, who had independently discovered the virus under the name "human T-lymphotropic virus type III". The virus, renamed "human immunodeficiency virus", was later shown to cause AIDS. Barré-Sinoussi has continued to research HIV, studying how the virus is transmitted from mother to child, the immune response to HIV, and how a small proportion of infected individuals, termed "long-term nonprogressors", can limit HIV replication without treatment. In 2008, she was awarded the Nobel Prize, with Montagnier, for the discovery of HIV.

In this month

Diagram of the bacteriophage MS2 capsid

1 April 1911: Peyton Rous showed that a cell-free isolate could transmit sarcoma in chickens, an early demonstration of cancer caused by a virus

7 April 1931: First electron micrograph taken by Ernst Ruska and Max Knoll

8 April 1976: Bacteriophage MS2 (pictured) sequenced by Walter Fiers and coworkers, first viral genome to be completely sequenced

8 April 1990: Death from AIDS of Ryan White, haemophiliac teenager for whom the Ryan White Care Act is named

8 April 1992: Tennis player Arthur Ashe announced that he had been infected with HIV from blood transfusions

9 April 1982: Stanley Prusiner proposed proteinaceous prions as the cause of scrapie

12 April 1955: Success of trial of Jonas Salk's polio vaccine announced

12 April 2013: New order of double-stranded DNA bacteriophages, Ligamenvirales, announced

15 April 1957: André Lwoff proposes a concise definition of a virus

21 April 1989: Discovery of hepatitis C virus by Qui-Lim Choo and colleagues

28 April 1932: First yellow fever vaccine announced at an American Societies for Experimental Biology meeting by Wilbur Sawyer

29 April 2015: PAHO and WHO declared the Americas region free from rubella transmission

30 April 1937: Discovery of Theiler's murine encephalomyelitis virus, later a model for multiple sclerosis research

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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