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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

British World War 2 poster, stressing the economic cost of the common cold

The common cold is an upper respiratory tract disease which primarily affects the nose, throat and sinuses, and occasionally the conjunctiva of the eyes. Over 200 viruses can cause colds, most commonly rhinoviruses but also coronaviruses, influenza viruses, adenoviruses and others. Adults catch an average of 2–5 colds a year and children 6–12, making it the most common human disease. The economic costs are huge, with colds responsible for 40% of time lost from work in the U.S. Colds are described in the Egyptian Ebers papyrus, the oldest surviving medical text, written before the 16th century BCE.

Symptoms include cough, sore throat, runny nose, nasal congestion, sneezing and muscle aches; fever is common in young children. Symptoms typically resolve in 7–10 days, although some can last up to 3 weeks. The immune response to infection, rather than tissue destruction by the virus, causes most of the symptoms. Transmission occurs via airborne droplets and by contact with nasal secretions or contaminated objects. Cold viruses can survive for prolonged periods in the environment (over 18 hours for rhinoviruses). Hand washing can help to prevent spread. No effective antiviral treatment or vaccine currently exists.

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A man sneezing

Transmission via the respiratory route is important for many viruses, including influenza, measles and varicella zoster virus.

Credit: James Gathany (2009)

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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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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

Host response: antibody • immune systemFeatured article • RNA interferenceFeatured article

Social & Media: And the Band Played On • Contagion • "Flu Season" • Frank's CockFeatured article • Race Against TimeFeatured article • social history of virusesFeatured article • "Steve Burdick" • "The Time Is Now"

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Selected virus

X-ray crystallographic structure of the Norwalk virus capsid

Noroviruses are a genus of non-enveloped, single-stranded RNA viruses in the Caliciviridae family. The positive-sense RNA genome is approximately 7.5 kbp. Known noroviruses fall into five different genogroups (GI–GV); three groups infect humans, the other two mice, cattle and other bovines. All are considered strains of a single species, Norwalk virus.

Noroviruses are extremely contagious, with fewer than 20 virus particles being infectious. They are transmitted directly from person to person and indirectly via contaminated water and food. After infection, the virus replicates in the small intestine, causing acute gastroenteritis, which develops 24–48 hours after exposure and lasts for 24–60 hours. The characteristic symptoms include nausea, forceful vomiting, watery diarrhoea and abdominal pain. Infection is usually self-limiting and rarely severe. Noroviruses are the most common cause of viral gastroenteritis in humans, affecting around 267 million people a year and causing over 200,000 deaths, mainly in less-developed countries and in very young, elderly or immunosuppressed people. Hand washing with soap and water is effective in reducing transmission.

Did you know?

HMAT Boonah

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.

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.

In this month

Red ribbon signifying solidarity with people living with HIV/AIDS

5 June 1981: First report of HIV/AIDS (symbol pictured) appeared in medical literature

6 June 1997: Gene silencing in plants shown to be a viral defence mechanism

7–13 June 1962: Donald Caspar and Aaron Klug proposed the quasi-equivalence principle of virus structure

7–13 June 1962: André Lwoff proposed a viral classification scheme based on nature of genome, type of symmetry and presence of envelope

7–13 June 1962: George Hirst proposed that the influenza virus genome is segmented

13 June 2012: First case of Middle East respiratory syndrome coronavirus (MERS-CoV) occurs in Saudi Arabia

21 June 1996: Nevirapine approved, first NNRTI for HIV/AIDS

28 June 2011: FAO declared rinderpest eradicated



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