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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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Light microscope image of the cervix, showing normal epithelium (right) and carcinoma in situ (left), a pre-cancerous precursor to cervical cancer

Cervical cancer is a tumour of the cervix, the junction between the uterus and vagina in the female reproductive tract. Certain types of human papillomavirus (HPV) are implicated in more than 90% of these cancers, although the great majority of HPV infections of the cervix are not associated with cancer. HPV is transmitted by vaginal sex, infecting cervical epithelial cells. In 5–10% of cases, infection persists for years, and pre-cancerous changes called cervical intraepithelial neoplasia can develop. These can regress, but sometimes progress to cancer. Although in nearly all forms of the cancer, HPV infection is considered essential for cancer to develop, other risk factors are involved, including smoking, HIV infection and other forms of immune suppression.

Cervical cancer is the fourth most common cancer in women worldwide. It can be detected by screening; screening every 3–5 years, with appropriate follow-up, can reduce cancer incidence by up to 80%. HPV vaccines protect against types 16 and 18, which cause three-quarters of cancers. Where screening and vaccination are not available, cervical cancer has substantial mortality; worldwide, an estimated 528,000 cases and 266,000 deaths occurred in 2012, with 80% of these being in developing countries.

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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)

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Ribbon diagram of HIV reverse transcriptase

Reverse transcriptase is an enzyme that makes complementary DNA from an RNA template. It was discovered in 1970 by Howard Temin in Rous sarcoma virus, a retrovirus that causes tumours in chickens, and independently the same year by David Baltimore, for which the two shared the Nobel Prize in Physiology or Medicine. The discovery was initially controversial, as reverse transcription contradicts the central dogma of molecular biology, that information flows from DNA to RNA to protein.

Reverse transcription is essential for the replication of retroviruses, and allows them to integrate into the host genome as a provirus. The enzyme is a target for reverse-transcriptase inhibitors, an important class of anti-HIV drugs. The process is also important in eukaryotic genomes, in the replication of chromosome ends and retrotransposons, a type of mobile genetic element. Reverse transcriptase is widely used in the laboratory for molecular cloning, RNA sequencing, polymerase chain reaction and genome analysis.

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

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

Diagram of HIV structure

Human immunodeficiency virus (HIV) is a lentivirus, an RNA virus in the retrovirus family. Two types of HIV have been characterised: HIV-1 is the more virulent and is responsible for most infections worldwide; HIV-2 is mainly confined to West Africa. The genome consists of two copies of a single-stranded +RNA, which contains nine genes. The roughly spherical virus particle has a diameter of about 120 nm; it is enveloped and contains a conical capsid made of around 2,000 copies of the p24 protein. The envelope glycoprotein, a trimeric complex of gp120 and gp41, binds to CD4, the primary receptor on the host cell.

HIV infects key cells in the human immune system including CD4+ helper T cells, macrophages and dendritic cells. Infection leads to low levels of CD4+ T cells via several mechanisms, resulting in a progressive immunodeficiency disease known as AIDS. Transmission occurs by the transfer of bodily fluids including blood, semen and breast milk, in which the virus is present both as free virus particles and within infected immune cells.

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Peanut plant (Arachis hypogaea)

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.

Selected intervention

Gardasil human papillomavirus vaccine

Several human papillomavirus (HPV) vaccines have been approved to protect against infections with HPV. Both Cervarix and Gardasil protect against the high-risk HPV types 16 and 18, which are associated with the majority of HPV-related cervical, anal, vaginal and oral cancers. Other high-risk HPV types exist; Cervarix and to a lesser extent Gardasil provide some degree of cross-protection against them. Gardasil is a quadrivalent vaccine that also protects against low-risk HPV-6 and -11, which are associated with most cases of genital warts. A second-generation nine-valent Gardasil vaccine protects against five additional high-risk HPV types. All three are subunit vaccines, containing only the L1 capsid protein of the virus, which self-assembles into virus-like particles. The optimal strategy for using these vaccines is not yet clear. Some advocate giving Gardasil to men and boys with the primary aim of protecting their female sexual partners; others consider vaccinating only women and girls to be more cost effective. The optimal age for vaccination is unknown.

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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