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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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Symptoms of dengue fever

Dengue fever, or breakbone fever, is an infectious tropical disease caused by the dengue virus. Symptoms include fever, headache, muscle and joint pains, and a characteristic skin rash that is similar to measles. Rarely, the disease develops into the life-threatening dengue hemorrhagic fever, resulting in bleeding, low levels of blood platelets and blood plasma leakage, or into dengue shock syndrome, where dangerously low blood pressure occurs.

Dengue is transmitted by several species of Aedes mosquito, mainly A. aegypti. The incidence of dengue fever has increased dramatically since the 1960s, with around 50–100 million people infected yearly; the virus is endemic in over 110 countries. The virus has four different types. Infection with one type usually gives lifelong immunity to that type but only short-term immunity to the others, and subsequent infection with a different type increases the risk of severe complications. Treatment of acute dengue is supportive, using either oral or intravenous rehydration for mild or moderate disease, and intravenous fluids and blood transfusion for more severe cases. There is no commercially available vaccine. Reducing the habitat and number of mosquitoes and limiting exposure to bites are the main methods of control.

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Gods slaandehand over Nederland, door pest-siekte onder het rund vee by Jacobus Eussen (1745)

Rinderpest was a Morbillivirus that caused catastrophic cattle plagues for centuries. It was declared the second virus to have been eradicated globally in 2011.

Credit: Jacobus Eussen (1745)

In the news

Electron micrograph of Zaire ebolavirus

8 August: WHO declares the ongoing West African outbreak of Ebola virus disease (virus pictured) to be an international public health emergency; since the outbreak began, there have been more than 1750 suspected cases and 961 deaths. WHO

20 July: A study in rhesus monkeys suggests that after rectal infection with simian immunodeficiency virus, a model for HIV infection, the virus reaches hard-to-treat reservoirs before it can be detected in the blood, posing a challenge for eradication. Nature

17 July: At least six HIV/AIDS researchers and activists travelling to the World AIDS Conference in Melbourne are killed when Malaysia Airlines Flight 17 crashes; confirmed casualties are Joep Lange (University of Amsterdam/Amsterdam Institute for Global Health and Development), Jacqueline van Tongeren (Amsterdam Institute for Global Health and Development), Pim de Kuijer and Martine de Schutter (Aids Fonds/STOP AIDS NOW!), Lucie van Mens (The Female Health Company) and Glenn Thomas (World Health Organisation). IAS

Electron micrograph of Lassa virus particles

11 July: Three injections of a tetravalent dengue vaccine confer better protection than placebo in a phase III trial among over 10,000 children in endemic areas of Asia. Lancet

6 July: Hundreds of novel bacteriophage species are discovered in human gut flora samples. Nat Biotechnol

27 June: Entry of Lassa virus (pictured) into susceptible cells is shown to be a two-step process, with the virus binding to a second receptor, LAMP1, located inside lysosomes. Science

25 June: The human monoclonal antibody m102.4 is an effective treatment for Nipah virus in an African green monkey model. Sci Transl Med

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Child receiving the oral polio vaccine

Two polio vaccines are used against the paralytic disease polio. Each vaccine has benefits and disadvantages. The first, developed by Jonas Salk, consists of inactivated poliovirus. Based on three wild virulent strains, inactivated using formalin, it is administered by injection. It confers IgG-mediated immunity, which prevents poliovirus from entering the bloodstream and protects the motor neurons, eliminating the risk of bulbar polio and post-polio syndrome. The second, developed by Albert Sabin, consists of three live virus strains, attenuated by growth in cell culture. They contain multiple mutations, stopping them from replicating in the nervous system. The Sabin vaccine provides longer-lasting immunity than the Salk vaccine, and can be administered orally, making it more suitable for mass vaccination campaigns. In around 1 in 750,000 people, the live vaccine reverts to a virulent form and causes paralysis. Vaccination has reduced the number of polio cases from around 350,000 in 1988 to just 223 in 2012, and eradicated the disease from most countries.

In this month

Diagram of the human rhinovirus capsid

3 September 1917: Discovery of bacteriophage of Shigella by Félix d'Herelle

8 September 1976: Death of Mabalo Lokela, the first known case of Ebola virus

11 September 1978: Janet Parker was the last person to die of smallpox

12 September 1985: Structure of human rhinovirus 14 (pictured) solved by Michael Rossmann and colleagues, the first atomic-level structure of an animal virus

26 September 1997: Combivir (zidovudine/lamivudine) approved; first combination antiretroviral

27 September 1985: Structure of poliovirus solved by Jim Hogle and colleagues

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Electron micrograph of canine parvovirus

Canine parvovirus type 2 is a non-enveloped single-stranded DNA virus in the Parvoviridae family. The icosahedral viral capsid is only 20–26 nm in diameter, making it one of the smallest viruses. The genome is about 5000 nucleotides long. The virus is very similar to feline panleukopenia virus, another parvovirus, as well as mink enteritis and raccoon and fox parvoviruses. It infects dogs, wolves, foxes and other canids, big cats and occasionally domestic cats, but cannot infect humans.

A relatively new disease, canine parvovirus 2 infection was first recognised in 1978 and rapidly spread worldwide. The virus is transmitted by direct or indirect contact with faeces. The virus replicates in the lymphoid tissue in the throat, then spreads to the bloodstream to infect cells of the lymph nodes, intestinal crypts and bone marrow. The disease has two different types: the more common intestinal form causes severe vomiting and dysentery; the cardiac form affects in puppies under 8 weeks, causing respiratory or cardiovascular failure. Mortality can reach 91% in untreated cases. An effective vaccine is available.

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Tobacco mosaic virus on a tobacco leaf, showing the characteristic mottling

Plant viruses face different challenges from animal viruses. As plants do not move, transmission between hosts often involves vectors, most commonly insects, but also fungi, nematodes and protozoa. Plant viruses can also spread via seeds, and by direct transfer of sap. Plant cells are surrounded by cell walls which are difficult to penetrate. Movement between cells occurs mainly by transport through plasmodesmata, and most plant viruses encode movement proteins to make this possible. Although plants lack an adaptive immune system, they have complex defences against viral infection. Plant viruses often cause disease, and are thought to cause up to US$60 billion losses to global crop yields each year.

Most plant viruses are rod-shaped, with protein discs forming a tube surrounding the viral genome; isometric particles are another common structure. They rarely have an envelope. The great majority have an RNA genome, which is usually small and single stranded. Plant viruses are grouped into 73 genera and 49 families. Tobacco mosaic virus (pictured) is among the best characterised of the 977 species officially recognised in 1999.

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A still life of flowers in a vase including striped tulips

Selected biography

Jonas Salk (1955)

Jonas Edward Salk (28 October 1914 – 23 June 1995) was an American medical researcher and virologist, best known for developing the first successful polio vaccine.

Unlike most other researchers, Salk focused on creating an inactivated or "killed" virus vaccine, for safety reasons. The vaccine he developed combines three strains of wild-type poliovirus, inactivated with formalin. The field trial that tested its safety and efficacy in 1954 was one of the largest carried out to date, with vaccine being administered to over 440,000 children. When the trial's success was announced, Salk was hailed as a miracle worker and national hero. A little over two years later, 100 million doses of the vaccine had been distributed throughout the US, with few reported adverse effects. An inactivated vaccine based on the Salk vaccine is the mainstay of polio control in many developed countries.

Salk also researched vaccines against influenza and HIV. In 1960, he founded the Salk Institute for Biological Studies research centre in La Jolla, California.

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