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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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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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Ribbon model of CCR5 (yellow), shown within the cell membrane (grey and red)

CCR5 is a human membrane protein that acts as a secondary receptor for HIV, enabling the viral and cell membranes to fuse. People with two copies of a mutated Δ32 form of CCR5 are naturally resistant to infection by most strains of HIV, and the normal form is the target of entry inhibitors such as maraviroc.

Credit: Thomas Splettstoesser (18 July 2012)

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

The structure of aciclovir (bottom) compared with guanosine (top)

Aciclovir (also acyclovir, Zovirax) is a nucleoside analogue, which mimics the nucleoside guanosine. Extremely selective and low in cytotoxicity, it was seen as the start of a new era in antiviral therapy. Aciclovir was discovered by Howard Schaffer and colleagues, and developed by Schaffer and Gertrude Elion, who was awarded the 1988 Nobel Prize in Medicine in part for its development. Nucleosides isolated from a Caribbean sponge, Cryptotethya crypta, formed the basis for its synthesis. Aciclovir differs from earlier nucleoside analogues in containing only a partial nucleoside structure: the sugar ring (pictured in blue) is replaced with an open chain (pink). One of the most commonly used antiviral drugs, aciclovir is active against most viruses in the herpesvirus family. It is mainly used to treat herpes simplex virus infections, chickenpox and shingles. Aciclovir resistance is rare.

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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Selected virus

Electron micrograph of two Epstein–Barr virus particles

Epstein–Barr virus (EBV) (also human herpesvirus 4) is a DNA virus in the Herpesviridae family which infects humans. The virion is around 120–180 nm in diameter. Like all herpesviruses, the capsid is surrounded by a protein tegument, as well as an envelope. The double-stranded DNA genome is about 192 kb with around 85 genes, making it one of the more complex viruses.

Transmission is in saliva and genital secretions. The virus infects epithelial cells in the mouth and pharynx and B cells of the immune system, producing virions by budding. EBV also becomes latent in B cells, possibly in the bone marrow, allowing the infection to persist lifelong. In the latent state, the linear genome is made circular and replicates separately from the host DNA as an episome. Reactivation is thought to be triggered by the B cell responding to other infections. EBV infection is almost ubiquitous. Infectious mononucleosis or glandular fever can occur when first infection is delayed until adolescence or adulthood. EBV is associated with some types of cancer, including Burkitt's lymphoma and nasopharyngeal carcinoma. In people with HIV, it can cause hairy leukoplakia and central nervous system lymphomas.

Selected article

Child receiving oral polio vaccine in India

Vaccination or immunisation is the administration of antigenic material (a vaccine) to stimulate an individual's immune system to develop adaptive immunity to a virus or other pathogen. The active agent of a vaccine may be intact but inactivated (non-infective) or attenuated (with reduced infectivity) forms of the pathogen, or purified components that have been found to be highly immunogenic, such as viral envelope proteins. Smallpox was the first disease for which a vaccine was produced, by Edward Jenner in 1796.

Vaccination is the most effective method of preventing infectious diseases and can also ameliorate the symptoms of infection. Widespread immunity due to mass vaccination campaigns is largely responsible for the worldwide eradication of smallpox and the restriction of diseases such as polio and measles from much of the world. Vaccination efforts have been met with some controversy since their inception, on scientific, ethical, political, medical safety, and religious grounds.

Did you know?

HMAT Boonah

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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