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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".

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Selected disease

Cow with a ruptured blister in the mouth, a sign of foot-and-mouth disease

Foot-and-mouth disease or FMD is an economically important disease of even-toed ungulates (cloven-hoofed animals) and some other mammals caused by the FMD virus, a picornavirus. Hosts include cattle, water buffalo, sheep, goats, pigs, antelope, deer and bison; human infection is extremely rare. After a 1–12-day incubation, animals develop high fever, and then blisters inside the mouth (pictured) and on the hooves, which can rupture and cause lameness. Weight loss and reduction in milk production are other possible long-term consequences. Mortality in adult animals is low (2–5%). The virus is highly infectious, with transmission occurring via direct contact, aerosols, semen, consumption of infected food scraps or feed supplements, and via inanimate objects including fodder, farming equipment, vehicles, standing water, and the clothes and skin of humans. Some infected ruminants can transmit infection as asymptomatic carriers.

Friedrich Loeffler showed the disease to be viral in 1897. FMD was widely distributed in 1945. By 2014, North America, Australia, New Zealand, much of Europe, and some South American countries were free of the disease. Major outbreaks include one in the UK in 2001 that cost an estimated £8 billion. The virus is highly variable, with seven serotypes. A vaccine is available, but protection is temporary and strain specific. Other control methods include monitoring programmes, trade restrictions, quarantine, and the slaughter of infected and healthy at-risk animals.

Selected image

Ribbon diagram showing T7 RNA polymerase (blue) making messenger RNA (green) based on a DNA template (orange)

Some viruses, such as the T7 bacteriophage, encode their own RNA polymerase, the enzyme that makes messenger RNA based on a DNA template. The T7 enzyme has a single subunit, and is more like chloroplast and mitochondrial enzymes than those of bacteria or the cell.

Credit: Thomas Splettstoesser (25 June 2007)

In the news

Map showing the distribution of coronavirus cases; black: highest incidence; dark red to pink: decreasing incidence; grey: no recorded cases
Map showing the distribution of coronavirus cases; black: highest incidence; dark red to pink: decreasing incidence; grey: no recorded cases

2 April: The ongoing pandemic of a novel coronavirus is accelerating rapidly; more than 900,000 confirmed cases, including more than 45,000 deaths, have been documented globally since the outbreak began in December 2019. WHO 1, 2

27 March: An international, randomised, non-blinded, clinical trial organised by the World Health Organization of four potential treatments for COVID-19remdesivir; chloroquine or hydroxychloroquine; lopinavir/ritonavir; or lopinavir/ritonavir plus interferon-beta – is about to start enrolling patients. Science, WHO

16 March: A phase I clinical trial of a messenger RNA-based vaccine candidate for the novel coronavirus begins in Seattle. NIH

11 March: The World Health Organization describes the ongoing outbreak of respiratory disease caused by a novel coronavirus as a pandemic. WHO

9 March: No new cases have been recorded in three weeks in the ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo; as of 3 March there had been a total of 3444 cases, including 2264 deaths, since the outbreak began in August 2018. WHO 1, 2

28 February: The World Health Organization raises its assessment of the global risk from the ongoing outbreak of respiratory disease caused by a novel coronavirus to "very high". WHO

False-coloured electron micrograph of novel coronavirus
False-coloured electron micrograph of novel coronavirus

12 February: The ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo remains a Public Health Emergency of International Concern, according to the World Health Organization. WHO 1

7 February: Chinese scientists announce that novel coronavirus (2019-nCoV) is 99% identical to a coronavirus isolated from pangolins, suggesting these animals might be an intermediate host. Nature

5 February: A study of 2658 samples from 38 different types of cancer found that 16% were associated with a virus, higher than previous estimates, but did not identify any new candidate tumour viruses. Nat Genet

4 February: Over 2500 putative circular DNA virus genomes are catalogued from metagenomic surveys of human and animal samples, including over 600 dissimilar to existing virus groups. eLife, Science

3 February: The US National Institute of Allergy and Infectious Diseases stops the South African HVTN 702 Phase IIb/III clinical trial of an investigational HIV vaccine early, after the vaccine failed to prevent HIV infection. NIH

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 groupings similar to those used for cellular organisms, which reflect viruses thought to have a common ancestor. As of 2019, one phylum, 6 classes, 14 orders, 143 families, 846 genera and 4,958 species of viruses have been defined. The majority of virus families have not yet been assigned to an order.

Selected outbreak

The masked palm civet (Paguma larvata) is thought to have been the source of SARS coronavirus

In the severe acute respiratory syndrome (SARS) outbreak, the first cases of the newly emerged SARS coronavirus were reported in November 2002 from the Chinese Guangdong province. The virus soon spread across Asia, with China, Hong Kong, Taiwan and Singapore being the worst affected countries; a secondary outbreak occurred in Canada. The rapid initial spread of the outbreak has been in part attributed to China's slow response to the early cases. Over 8,000 people were infected, with a case fatality rate of 11%. Those over 65 years had a much higher mortality rate, greater than 55%. The outbreak was contained by July 2003, and no cases have been reported since 2004.

At the time of the outbreak, the immediate source of SARS coronavirus was thought to have been the masked palm civet (Paguma larvata; pictured), which was sold as food in Guangdong markets. The virus was also found in raccoon dogs, ferret badgers and domestic cats. More recent research has suggested that the natural reservoir could be horseshoe bats.

Selected quotation

Michael Kirby on the cost of antiviral drugs

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Selected virus

Structure of adeno-associated virus serotype 2

Adeno-associated viruses (AAVs) are two small DNA viruses in the Dependoparvovirus genus of the Parvoviridae family. They cannot complete their lytic replication cycle without a helper virus, which include adenoviruses, herpesviruses and vaccinia. In the absence of the helper, AAVs can integrate into the host genome at a specific site on human chromosome 19, or persist as an episome. The 20 nm icosahedral capsid lacks an envelope, and contains a single-stranded DNA genome of around 4.7 kb. AAVs infect humans and some other primates without causing disease. They generate only a mild immune response, including neutralising antibodies. The best-studied of the 11 serotypes, AAV-2, infects nerve cells, liver cells, skeletal muscle and vascular smooth muscle, using heparan sulphate proteoglycan as its primary receptor.

Its low pathogenicity makes AAV an attractive basis for viral vectors for gene therapy. Alipogene tiparvovec to treat lipoprotein lipase deficiency was the first gene therapy to be licensed, but was later withdrawn. Promising results have been obtained in early clinical trials with AAV-based gene therapy in haemophilia, congestive heart failure, spinal muscular atrophy, Parkinson's disease and the rare eye disease Leber congenital amaurosis.

Did you know?

Peanut plant (Arachis hypogaea)

Selected biography

Peter Piot in 2006

Peter Piot (born 17 February 1949) is a Belgian virologist and public health specialist, known for his work on Ebola virus and HIV.

During the first outbreak of Ebola in Yambuku, Zaire in 1976, Piot was one of a team that discovered the filovirus in a blood sample. He and his colleagues travelled to Zaire to help to control the outbreak, and showed that the virus is transmitted via blood and during preparation of bodies for burial. He advised WHO during the West African Ebola epidemic of 2014–16.

In the 1980s, Piot participated in collaborative projects in Burundi, Côte d'Ivoire, Kenya, Tanzania and Zaire, including Project SIDA in Kinshasa, the first international project on AIDS in Africa, which provided the foundations for understanding HIV infection in that continent. He was the founding director of UNAIDS, and has served as president of the International AIDS Society and assistant director of the WHO Global HIV/AIDS Programme. As of 2020, he directs the London School of Hygiene & Tropical Medicine.

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. Oseltamivir can reduce the duration of influenza symptoms by 0.5–1 days. 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.



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