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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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Child with a deformed right leg due to poliomyelitis

Poliomyelitis, also called polio or infantile paralysis, was one of the most feared childhood diseases of the 20th century. It has caused paralysis and death for much of human history. The causative agent, poliovirus, spreads via the faecal–oral route. Infections are usually asymptomatic, but in about 1% of cases, the virus enters the central nervous system and preferentially infects and destroys motor neurons, leading to muscle weakness and acute flaccid paralysis. Spinal polio is the most common form, accounting for nearly 80% of cases of paralytic polio; it is characterized by asymmetric paralysis usually involving the legs. Bulbar involvement is rare, but in severe cases the virus can affect the phrenic nerve and prevent breathing, so that patients require mechanical ventilation with an iron lung or similar device.

Polio had existed for thousands of years as an endemic pathogen until the 1880s, when major epidemics began to occur in Europe and later the United States. Polio vaccines developed in the 1950s have reduced the annual incidence from many hundreds of thousands to under a thousand, and as of 2013, a global eradication campaign is ongoing.

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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)

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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.

In the news

Electron micrograph of Zaire ebolavirus

24 March: A small study of a multivalent norovirus virus-like particle vaccine shows a broad antibody response is generated, which covers novel virus variants. PLOS Med

24 March: An Ebola vaccine (virus pictured) based on the 2014 strain is shown to be safe and to generate an immune response in a phase I clinical trial in China. Lancet

23 March: A novel virus, ANMV-1, believed to infect anaerobic archaea in a deep-sea methane seep, is shown to have the first diversity-generating retroelement found in archaea or their viruses, which has the potential to generate rapid genetic diversity in the virus. Nat Comm

22 March: The lowest weekly total of new cases of Ebola virus disease in 2015 has been recorded in the ongoing West African outbreak; since the outbreak began, there have been nearly 25,000 suspected cases and 10,326 deaths. WHO

18 March: A test to distinguish viral from bacterial infections is developed, based on TRAIL (pictured) and other host proteins induced after infection. PLOS ONE

10 March: In the ongoing outbreak of Middle East respiratory syndrome coronavirus (MERS-CoV; pictured), cases continue to be reported in Saudi Arabia; since the outbreak started in September 2012, there have been 1075 cases with at least 404 deaths. WHO

Ribbon diagram of TRAIL

9 March: Real-time imaging of SIV in macaques using immuno-PET reveals unexpectedly high levels of virus in the nasal cavity, lung and male genital tract in antiretroviral-treated animals. Nat Meth

4 March: The International Committee on Taxonomy of Viruses formally ratify their 2014 species list, with one new family of double-stranded DNA bacteriophages, Sphaerolipoviridae, one new subfamily of Myoviridae, Eucampyvirinae, fifty new genera and 359 new species recognised. ICTV

3 March: Four RNA viruses found in farmed honeybees – acute bee paralysis, black queen cell, deformed wing and slow bee paralysis viruses – are widespread among wild bumblebee species in a survey across Britain. J Anim Ecol

2 March: A 3D image of Acanthamoeba polyphaga mimivirus at 125 nm resolution is constructed from multiple high-energy X-ray diffraction patterns of single virions, rather than crystals, in the first application of this technique to a virus. Nature

25 February: H7N9 avian influenza infection continues in China, with 59 cases reported since 21 January. WHO

25 February: WHO calls for increased measles vaccination coverage in the light of outbreaks across Europe; the ongoing outbreak in North America continues. WHO,CDC

Graphic of Middle East respiratory syndrome coronavirus

24 February: A clinical trial in England shows that pre-exposure prophylaxis with tenofovir/emtricitabine (Truvada) reduces the risk of HIV infection by 86% in gay men engaging in high-risk sex. CROI

18 February: Varicella zoster virus antigens are found in 74% of giant cell arteritis biopsies but only 8% of normal ones, suggesting the virus might have a role in triggering disease. Neurology

29 January: A total of 112 novel negative-sense RNA viruses of arthropods, including a putative new family of circular RNA viruses, are identified in a study of 70 arthropod species in China. e-Life

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Viruses & Subviral agents: elephant endotheliotropic herpesvirus • HIV • introduction to virusesFeatured article • Playa de Oro virus • poliovirus • prion • rotavirusFeatured article • virusesFeatured article

Diseases: colony collapse disorder • common cold • dengue feverFeatured article • gastroenteritis • hepatitis B • hepatitis C • herpes simplex • herpes zoster • HIV/AIDS • influenzaFeatured article • meningitisFeatured article • poliomyelitisFeatured article • smallpox

Epidemiology & Interventions: 1918 flu pandemic • 2007 Bernard Matthews H5N1 outbreak • 2009 flu pandemic • polio vaccine

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Selected virus

Electron micrograph of Sin Nombre virus, a hantavirus

Hantaviruses are a genus of RNA viruses in the Bunyaviridae family. The enveloped virion is 120–160 nm in diameter and contains a single-stranded –RNA genome with three segments. They infect many different species of rodent and can be transmitted to humans, in some cases causing serious disease. Hantaan River virus, the first known hantavirus, was discovered in 1976 as the cause of a novel haemorrhagic fever affecting combatants in the Korean War. Hantavirus haemorrhagic fever with renal syndrome, as the disease is now known, can also be caused by other hantaviruses, including Dobrava-Belgrade virus. Some hantaviruses, including Sin Nombre and Bayou, cause a pulmonary syndrome. Others have not yet been associated with disease.

Unlike other bunyaviruses, hantaviruses are not transmitted by arthropods. Rodents act as the vector, with transmission to humans usually occurring via contact with urine, saliva or faeces, by inhalation of aerosolised excreta or by bite. Little is understood about how hantaviruses cause disease; the main site of viral replication in the body is unknown. Rodent control is important in disease prevention.

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Baby Asian elephant

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.

Selected intervention

Ball-and-stick model of zidovudine

Zidovudine (ZDV) (also known as AZT and Retrovir) is an antiretroviral drug used in the treatment of HIV/AIDS. Classed as a nucleoside analogue reverse-transcriptase inhibitor, it inhibits HIV's reverse transcriptase enzyme, which copies the viral RNA into DNA and is essential for its replication. The first breakthrough in AIDS therapy, ZDV was licensed in 1987. While it significantly reduces HIV replication, leading to clinical and immunological benefits, when used alone ZDV does not completely stop replication, allowing the virus to become resistant to it. The drug is therefore used together with other anti-HIV drugs in combination therapy called highly active antiretroviral therapy (HAART). To simplify its administration, ZDV is included in Combivir, Trizivir and other combination pills. ZDV can also be used to prevent HIV transmission, such as from mother to child during childbirth or after a needlestick injury.

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

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



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