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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 image

Portrait of Louis Pasteur by Albert Edelfelt (1885)

Louis Pasteur invented a vaccine against rabies, and tested it on a boy bitten by a rabid dog in 1885.

Credit: Albert Edelfelt (1885)

Selected article

Cases of poliomyelitis in 2005 (top) and 2017 (bottom). Red: endemic; orange: re-established; green: imported; blue: vaccine derived; grey: none
Cases of poliomyelitis in 2005 (top) and 2017 (bottom). Red: endemic; orange: re-established; green: imported; blue: vaccine derived; grey: none

A global drive to eradicate poliovirus started in 1988, when there were an estimated 350,000 cases of wild poliovirus infection globally. Two diseases, both caused by viruses, have been eradicated, smallpox in 1980 and rinderpest in 2011. Poliovirus only infects humans. It persists in the environment for a few weeks at room temperature and a few months at 0–8 °C. The oral polio vaccine is inexpensive, highly effective and is predicted to generate lifelong immunity.

Annual cases of wild poliovirus infection have declined steadily since 2013, with 22 reported cases in 2017. As of 2018, the virus remains endemic in only three countries: Afghanistan, Pakistan and Nigeria. Reversion of live vaccine strains to virulence has resulted in occasional cases of vaccine-associated paralysis. A lack of basic health infrastructure and civil war remain significant obstacles to eradication. Some local communities have opposed immunisation campaigns, and vaccination workers have been murdered in Pakistan and Nigeria.

In the news

False-coloured micrograph of Middle East respiratory syndrome coronavirus

22 June: Vaccine-derived poliovirus type 1 is confirmed to be circulating in Morobe Province, Papua New Guinea, after the country was declared free of the virus in 2000. WHO

16 June: An outbreak of Rift Valley fever has been confirmed in northern Kenya, with 26 human cases mainly in Wajir County, including 6 deaths, as well as widespread deaths and abortions in camels, goats and other livestock. WHO

1 June: In the ongoing outbreak of Nipah virus in Kerala state, south India, there have been 18 confirmed cases with 17 deaths in the Kozhikode and Mallapuram districts. WHO

31 May: In the ongoing outbreak of Middle East respiratory syndrome coronavirus (pictured), 75 cases have been reported in Saudi Arabia since 12 January, including 23 deaths. WHO

23 May: An outbreak of measles is ongoing in the Amazonas and Roraima states of Brazil, with 995 suspected cases, including two deaths. WHO

21 May: Vaccination with rVSV-ZEBOV starts in the Ebola virus outbreak in the Democratic Republic of the Congo; the outbreak has spread from rural Bikoro to the city of Mbandaka, with a total of 58 suspected cases reported, including 27 deaths. WHO

17 May: The sialic acid-bearing cellular receptor for influenza A virus is shown to be the voltage-gated calcium channel, Cav1.2 (pictured). Cell Host & Microbe

Ribbon diagram of Cav1.2

16 May: The cellular receptor for several alphaviruses associated with rheumatic disease, including chikungunya, Mayaro, O'nyong nyong and Ross River viruses, is shown to be the cell adhesion molecule Mxra8. Nature

14 May: IMP-1088, a compound targeting human N-myristoyltransferases NMT1 and NMT2, is shown to prevent three different picornaviruses, rhinovirus, poliovirus and foot-and-mouth disease virus, from assembling capsids in vitro, by inhibiting myristoylation of the picornaviral VP0 protein. Nat Chem

9 May: Hepatitis B virus sequences are recovered from Bronze Age human remains up to 4,500 years old, and the virus is estimated to have evolved 8,600–20,900 years ago, disproving the hypothesis that it originated in the New World and spread to Europe in around the 16th century. Nature

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. Over 8,000 people were infected, with nearly 10% dying. Those over 50 years had a much higher mortality rate, approaching half. The outbreak was contained by July 2003.

The immediate source of SARS coronavirus is likely to have been the masked palm civet (Paguma larvata), which was sold as food in Guangdong markets. The virus was also found in raccoon dogs, ferret badgers and domestic cats, and closely related coronaviruses have been isolated from bats, which probably form the natural reservoir. The rapid initial spread of the outbreak has been in part attributed to China's slow response to the early cases.

Selected quotation

Michael Kirby on the cost of antiviral drugs

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Selected virus

Electron micrograph of tobacco mosaic virus

Tobacco mosaic virus (TMV) is an RNA virus in the Virgaviridae family that infects a wide range of plants, including tobacco, tomato, pepper, other members of the Solanaceae family, and cucumber. The rod-shaped virus particle is around 300 nm long and 18 nm in diameter, and consists of a helical capsid made from 2130 copies of a single coat protein, which is wrapped around a positive-sense single-stranded RNA genome of 6400 bases. The coat protein and RNA can self-assemble to produce infectious virus.

Infection often causes characteristic patterns, such as "mosaic"-like mottling and discoloration on the leaves. TMV causes an economically important disease in tobacco plants. Transmission is frequently by human handling, and prevention of infection involves destroying infected plants, hand washing and crop rotation to avoid contaminated soil. TMV is one of the most stable viruses known. The fact that it does not infect animals and can readily be produced in gramme amounts has led to its use in numerous pioneering studies in virology and structural biology. TMV was the first virus to be discovered and the first to be crystallised.

Did you know?

Clara Maass

Selected biography

Rosalind Elsie Franklin (25 July 1920 – 16 April 1958) was a British biophysicist and X-ray crystallographer who made critical contributions to the understanding of the fine molecular structures of DNA, RNA and viruses.

Franklin led pioneering research on the structure of tobacco mosaic virus (TMV), a rod-like RNA virus, using X-ray crystallography. She first showed that, contrary to contemporary opinion, TMV virus particles were all of the same length. With Kenneth Holmes, she showed the virus's coat is composed of protein molecules arranged in helices. She designed and built a model of the virus to be exhibited at the 1958 World's Fair. She speculated that the virus is hollow, and correctly hypothesized that the RNA of TMV is single-stranded. Her work, together with that of Donald Caspar, revealed that the viral RNA is wound along the inner surface of the hollow virus. Her laboratory, which also included Aaron Klug, studied other plant viruses, including turnip yellow mosaic virus and viruses infecting potato, tomato and pea. Franklin also worked on icosahedral animal viruses, including poliovirus.

Franklin is commemorated in the Rosalind Franklin University of Medicine and Science.

In this month

Ball-and-stick model of raltegravir

6 October 2008: Nobel Prize in Physiology or Medicine awarded to Harald zur Hausen for showing that human papillomaviruses cause cervical cancer, and to Françoise Barré-Sinoussi and Luc Montagnier for discovering HIV

7 October 2005: 1918 Spanish influenza pandemic strain reconstituted

9 October 1991: Didanosine was the second drug approved for HIV/AIDS

12 October 1928: First use of an iron lung in a poliomyelitis patient

12 October 2007: Raltegravir (pictured) approved; first HIV integrase inhibitor

14 October 1977: Habiba Nur Ali was the last person to die from naturally occurring smallpox

14 October 2010: Rinderpest eradication efforts announced as stopping by the UN

16 October 1975: Last known case of naturally occurring Variola major smallpox reported

25 October 2012: Alipogene tiparvovec, a gene therapy for lipoprotein lipase deficiency using an adeno-associated virus-based vector, was the first gene therapy to be licensed

26 October 1977: Ali Maow Maalin developed smallpox rash; the last known case of naturally occurring Variola minor smallpox

26 October 1979: Smallpox eradication in the Horn of Africa formally declared by WHO, with informal declaration of global eradication

27 October 2015: Talimogene laherparepvec was the first oncolytic virus to be approved by the FDA to treat cancer

Selected intervention

Ball-and-stick model of ribavirin

Ribavirin is a nucleoside analogue, which mimics the nucleosides adenosine and guanosine. It is active against a wide range of DNA and RNA viruses, including influenza virus, herpes simplex virus, yellow fever, hepatitis C, West Nile, dengue fever and other flaviviruses, and is the only known treatment for the viruses causing viral haemorrhagic fevers. First synthesised in 1970 by Joseph T. Witkowski, ribavirin was originally developed as an anti-influenza drug, but failed to gain approval for this indication in the US. It has been used in an aerosol formulation against respiratory syncytial virus-related diseases in children. Ribavirin's main current use is against hepatitis C, in combination with pegylated interferon. Clinical use is limited by the drug building up in red blood cells to cause haemolytic anaemia. Its derivative and prodrug taribavirin, currently in clinical development, shows a similar spectrum of antiviral activity with reduced toxicity.



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