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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

Chickenpox rash in an adult male

Chickenpox is an infectious disease caused by primary infection with varicella zoster virus, a DNA virus of the Alphaherpesvirinae subfamily. The virus infects humans and some other primates and is transmitted by the respiratory route. Symptoms appear 10–21 days after exposure: an itchy vesicular skin rash mainly concentrated on the torso and head, and small ulcers in the oral cavity and tonsil areas. After the rash resolves, the virus remains latent in nerve cell bodies, and can emerge years or decades later to cause shingles.

A classic disease of childhood, the highest prevalence occurs at 4–10 years. Chickenpox is rarely fatal in people with a normal immune system, with around 6,800 deaths worldwide in 2010. Adults tend to experience more severe symptoms than children, and are at higher risk of complications such as pneumonia, hepatitis and encephalitis. Pregnant women and people with a suppressed immune system have the highest risk of serious complications. Chickenpox during the first 28 weeks of gestation can lead to foetal malformations. Infection in adults is usually treated with antiviral drugs, such as aciclovir or valaciclovir, which reduces both symptom severity and the risk of complications. A vaccine is available.

Selected picture

Chikungunya virus structure, based on cryoelectron microscopy

Chikungunya virus is an alphavirus transmitted by Aedes mosquitoes. The disease can cause severe joint pain, sometimes lasting for several months. Outbreaks have occurred across Africa, Asia and India, and in 2013–14, in South America and the Caribbean.

Credit: A2-33 (8 December 2013)

In the news

Electron micrograph of Zaire ebolavirus

7 April: The outbreak of Ebola haemorrhagic fever (virus pictured) in West Africa continues; there have been over 175 suspected cases including 105 deaths in Guinea, Liberia and Mali. WHO

27 March: South-east Asia becomes the fourth of the six WHO regions to be certified free from poliomyelitis. GPEI

21 March: The ongoing outbreak of poliomyelitis in Syria spreads to Iraq, in the first case there since 2000. WHO

19 March: A meta-analysis of over 29,000 patients hospitalised with H1N1 influenza during the 2009–10 pandemic shows that neuraminidase inhibitors reduced mortality in adults. Lancet Resp Med

14 March: The outbreak of chikungunya continues with 12,000 probable cases across French Guyana and ten Caribbean countries. ECDC

Ribbon diagram showing CCR5 in the cell membrane

12 March: The Middle East respiratory syndrome coronavirus (MERS-CoV) outbreak continues, with three cases in Saudi Arabia and the United Arab Emirates reported during March; since the outbreak started in September 2012, there have been 189 cases with 82 deaths. WHO

8 March: H7N9 avian influenza outbreak continues, with 13 cases reported in China during March. WHO

6 March: A preliminary trial in 12 HIV-positive people shows that genetically engineering the host's own T cells to express the HIV-resistant CCR5-Δ32 mutation (CCR5 pictured) is safe and feasible; the modified T cells persist several months and appear protected from infection. BBC

3 March: A viable Pithovirus specimen (pictured), the largest nucleocytoplasmic large DNA virus yet found, is discovered in 30,000-year-old samples of frozen tundra. Nature

Drawing of Pithovirus sibericum

1 March: A bomb attack kills at least 11 polio vaccine workers in north-west Pakistan; more than 40 polio vaccinators have been killed in the country since December 2012. BBC

24 February: A novel polio-like paralytic syndrome, possibly associated with enterovirus 68, has been reported in 20 people in California. BBC

19 February: A study in Britain and the Isle of Man suggests honey bees might be transmitting the deformed wing virus to wild bumblebees. BBC

Selected intervention

The structure of aciclovir (bottom) compared with guanosine (top)

Aciclovir (also acyclovir, Zovirax) is a nucleoside analogue, which mimics the nucleoside guanosine. 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 (pictured in blue) is replaced with an open chain (pink). 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.

Selected quotation

...the variety of genes on the planet in viruses exceeds, or is likely to exceed, that in all of the rest of life combined.

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Selected virus

Poliovirus (blue and pink) binding its cellular receptor, CD155 (purple)

Poliovirus is a human enterovirus, an RNA virus in the Picornaviridae family, associated with the paralytic disease poliomyelitis. The icosahedral virus particle is about 30 nanometres in diameter and lacks an envelope. It contains a relatively short, single-stranded positive RNA genome of around 7500 nucleotides, which encodes about ten viral products. The virus has a fairly high mutation rate even for an RNA virus. There are three serotypes, each with a slightly different capsid protein; PV1 is the most common.

The virus only infects humans; 95% of infections are asymptomatic. Infection occurs via the faecal–oral route and viral replication occurs in the alimentary tract. The virus enters the host cell by binding to an immunoglobulin-like receptor, CD155. Fully assembled poliovirus leaves the cell 4–6 hours after initiation of infection. Poliovirus was first isolated in 1909 by Karl Landsteiner and Erwin Popper. Its genome was sequenced in 1981. Among the simplest clinically significant viruses, poliovirus is one of the best-characterised viruses, and has become a useful model for studying RNA viruses.

Selected article

Martinus Beijerinck in his laboratory in 1921

Although vaccines protecting against viral infections were pioneered in the late 18th century, the history of virology is usually considered to begin over a century later. The first evidence for the existence of viruses came from experiments using filters with pores small enough to retain bacteria. Dmitry Ivanovsky showed in 1892 that sap from a diseased tobacco plant remained infectious despite having been filtered; this agent, later known as tobacco mosaic virus, was the first virus to be demonstrated. In 1898, Friedrich Loeffler and Paul Frosch showed that foot-and-mouth, an animal disease, was caused by a filterable agent. That year, Martinus Beijerinck (pictured) called the filtered infectious substance a "virus" – often considered to mark the beginning of virology, the scientific study of viruses and the infections they cause.

In 1926, Thomas Milton Rivers defined viruses as obligate parasites. Viruses were demonstrated to be particles, rather than a fluid, by Wendell Meredith Stanley in the 1930s.

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Transgenic mouse and litter

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. He identified the virus in the case of Janet Parker, who died from laboratory-acquired smallpox. His other research included influenza, coxsackie A, coxsackie B and hepatitis B viruses.

In this month

Louis Pasteur before 1985

1 July 1796: Edward Jenner first challenged James Phipps with variolation, showing that cowpox inoculation is protective against smallpox

3 July 1980: Structure of southern bean mosaic virus solved by Michael Rossmann and colleagues

6 July 1885: Louis Pasteur (pictured) gave rabies vaccine to Joseph Meister

10 July 1797: Jenner submitted paper on Phipps and other cases to the Royal Society; it was read to the society but not published

14–20 July 1968: First International Congress for Virology held in Helsinki

16 July 2012: FDA approved tenofovir/emtricitabine (Truvada) for prophylactic use against HIV; first prophylactic antiretroviral

19 July 2013: Pandoravirus described, with a genome twice as large as Megavirus

24–30 July 1966: International Committee on Nomenclature of Viruses (later the ICTV) founded

25 July 1985: Film star Rock Hudson made his AIDS diagnosis public, increasing public awareness of the disease

28 July 2010: First global World Hepatitis Day



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