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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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Smallpox rash

Smallpox is an infectious disease of humans caused by the Variola major and V. minor viruses. V. major causes a more serious disease with a mortality rate of 30–35%; V. minor is associated with milder symptoms and below 1% mortality. The virus is mainly transmitted by the respiratory route. Smallpox localises in small blood vessels of the skin and in the mouth and throat. In the skin, this results in a characteristic maculopapular rash, and later, raised fluid-filled blisters. Long-term complications of V. major infection include characteristic scars, commonly on the face, which occur in 65–85% of survivors. Blindness resulting from corneal ulceration and scarring, and limb deformities due to arthritis and osteomyelitis are less common complications, seen in about 2–5% of cases.

Smallpox probably emerged in human populations in about 10,000 BC; the mummified body of Egyptian pharaoh Ramses V shows evidence of smallpox rash. The disease was responsible for an estimated 300–500 million deaths during the 20th century. Smallpox vaccine, the earliest vaccine, was developed in the 18th century, and intensive vaccination campaigns led to smallpox being declared the first infectious disease to be eradicated globally in 1979.

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Culex mosquito larvae

Culex species mosquitoes transmit West Nile virus. Elimination of the stagnant water pools where the mosquitoes breed, together with other mosquito control measures, is key to preventing disease.

Credit: James Gathany (28 February 2006)

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

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

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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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.

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16th-century Aztec print showing a person with measles

Viruses have infected plants and animals, including humans, for millions of years. Epidemics caused by viruses began when human behaviour changed during the Neolithic period. Previously hunter-gatherers, humans developed more densely populated agricultural communities, which allowed viruses to spread rapidly and subsequently to become endemic. Louis Pasteur and Edward Jenner were the first to develop vaccines to protect against viral infections, long before viruses were discovered. The sizes and shapes of viruses remained unknown until the invention of the electron microscope in the 1930s, when the science of virology gained momentum. In the 20th century, many diseases were found to be caused by viruses.

Viruses are the most abundant biological entity on Earth. Although scientific interest in them arose because of the diseases they cause, most viruses are beneficial. They have driven evolution by transferring genes across species, play important roles in ecosystems, and are essential to life.

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Atlantic salmon

Selected biography

Randy Shilts (8 August 1951 – 17 February 1994) was an American journalist, author and AIDS activist. The first openly gay reporter for a mainstream US newspaper, Shilts covered the unfolding story of AIDS and its medical, social, and political ramifications from the first reports of the disease in 1981. New York University's journalism department later ranked his 1981–85 AIDS reporting in the top fifty works of American journalism of the 20th century. His extensively researched account of the early days of the epidemic in the US, And the Band Played On: Politics, People, and the AIDS Epidemic, first published in 1987, brought him national fame. The book won the Stonewall Book Award and was made into an award-winning film. Shilts saw himself as a literary journalist in the tradition of Truman Capote and Norman Mailer. His writing has a powerful narrative drive, and interweaves personal stories with political and social reporting.

He received the 1988 Outstanding Author award from the American Society of Journalists and Authors, the 1990 Mather Lectureship at Harvard University, and the 1993 Lifetime Achievement Award from the National Lesbian and Gay Journalists' Association. He died of AIDS in 1994.

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