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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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Shingles rash on the chest

Shingles, or herpes zoster, is a painful skin rash with blisters that, characteristically, occurs in a stripe limited to just one side of the body. The rash usually heals within 2–5 weeks, but around one in five people experience residual nerve pain for months or years.

Shingles is caused by varicella zoster virus (VZV), an alpha-herpesvirus. Initial VZV infection usually occurs in childhood causing chickenpox. After this resolves, the virus is not eliminated from the body, but remains latent in the nerve cell bodies of the dorsal root or trigeminal ganglia, without causing symptoms. Years or decades later, shingles occurs when virions in a single ganglion reactivate, travel down nerve fibres and infect the skin around the nerve. The shingles rash is restricted to the area of skin supplied by a single spinal nerve, termed the dermatome. Exactly how VZV remains latent in the body, and subsequently reactivates, is unclear.

Around a third of the population will develop shingles. Repeated episodes are rare. In the United States, about half the cases occur in people aged 50 years or older. Vaccination at least halves the risk, and prompt treatment with aciclovir or related antiviral drugs can reduce the severity and duration of the rash.

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False-coloured electron micrograph of the mosquito salivary gland, showing Eastern equine encephalitis virus particles in red

Eastern equine encephalitis virus is an Alphavirus that is transmitted between birds and mammals, including humans and horses, by several mosquito species. The virus (coloured in red) is found in the mosquito salivary gland, and is injected into the new host when the insect feeds.

Credit: Fred Murphy, Sylvia Whitfield, CDC (1968)

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Prion protein in its properly folded form

A prion is an infectious agent believed to be composed entirely of protein. This is in contrast to viruses and other known infectious agents, which all contain one or both of the nucleic acids, DNA and RNA. Prions propagate by transmitting a misfolded protein state. The prion induces existing, properly folded proteins in the host to convert into the misfolded prion form. This triggers a chain reaction resulting in large amounts of the prion form, disrupting cell function and causing cell death.

Prions are responsible for the transmissible spongiform encephalopathies in mammals. Human prion diseases include Creutzfeldt–Jakob disease, Gerstmann–Sträussler–Scheinker syndrome, fatal familial insomnia and kuru. Prion diseases of other mammals include bovine spongiform encephalopathy ("mad cow disease") in cattle and scrapie in sheep. All known mammalian prion diseases affect the structure of the brain or other neural tissue. All are currently untreatable and universally fatal. Proteins showing prion-type behaviour are also found in some fungi. Fungal prions do not appear to cause disease in their hosts.

In the news

Diagram of African swine fever virus

14 May: In the ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo – now the second-largest Ebola outbreak in history – new cases continue to increase, with 350 reported in the past 3 weeks, and a total of 1739 cases, including 1147 deaths, since the outbreak began in August 2018. WHO

3 May: In the ongoing Rift Valley fever outbreak in the Mayotte Islands in the Comoro group there have been 129 confirmed cases since the outbreak started in November 2018. WHO

2 May: A European observational study in 972 gay male couples finds no HIV transmission with unprotected sex where the HIV-positive partner's virus is fully suppressed by antiretroviral therapy. Lancet

25 April: A major outbreak of African swine fever ongoing in pigs in China since August 2018 has caused the loss of at least 40 million pigs, and the virus (pictured) has also been reported elsewhere in Southeast Asia. BBC

15 April: The directors of WHO and UNICEF warn that the more than 110,000 measles cases reported globally in January–March represent a nearly threefold increase over the same period in 2018. CNN

14 April: In the ongoing chikungunya virus outbreak in Congo, 6,149 suspected cases have been reported since the outbreak began in January, with nearly half in Kouilou Department. WHO

29 March: The filamentous bacteriophage Pf is shown to increase the pathogenicity of its bacterial host, Pseudomonas aeruginosa, an important human pathogen, by protecting it from the immune system in a mouse model. Science

12 March: The plant nanovirus, faba bean necrotic stunt virus – which has a segmented (multi-part) genome, with each of the eight segments being packaged separately – is shown to be able to replicate successfully even when its DNA segments do not all enter the same cell. eLife

8 March: The International Committee on Taxonomy of Viruses ratifies an update to virus classification, creating the Riboviria taxon for all RNA viruses at the new rank of realm. ICTV

5 March: Another case of apparent clearance of HIV from an infected patient after stem-cell therapy is reported. Nature

5 March: A Danish cohort study in more than 650,000 children confirms that vaccination with the measles, mumps and rubella (MMR) vaccine is not associated with autism. Ann Intern Med

20 February: Influenza A viruses that infect bats are shown to use a novel entry route, via the MHC class II HLA-DR isotype, rather than sialic acid. Nature

Selected outbreak

American soldiers with influenza H1N1 at a hospital ward at Camp Funston

The 1918–20 influenza pandemic, the first of the two involving H1N1 influenza virus, was unusually deadly. It infected 500 million people across the entire globe, with a death toll of 50–100 million (3–5% of the world's population), making it one of the deadliest natural disasters of human history. It has also been implicated in the outbreak of encephalitis lethargica in the 1920s. Despite the nickname "Spanish flu", the pandemic's geographic origin is unknown.

Most influenza outbreaks disproportionately kill young, elderly or already weakened patients; in contrast this predominantly killed healthy young adults. Contemporary medical reports suggest that malnourishment, overcrowded medical facilities and poor hygiene promoted fatal bacterial pneumonia. Some research suggests that the virus might have killed through a cytokine storm, an overreaction of the body's immune system. This would mean the strong immune reactions of young adults resulted in a more severe disease than the weaker immune systems of children and older adults.

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Selected virus

False-coloured electron micrograph of Sputnik virophage

Sputnik virophage is a DNA virus, discovered in 2008, that infects Acanthamoeba protozoa. It is a satellite virus of the giant mamavirus. It requires mamavirus to infect the cell simultaneously to replicate, hijacking the virus factories that mamavirus creates and impairing its replication. Sputnik was the first satellite to be shown to inhibit the replication of its associated helper virus. Such viruses have been termed "virophages" or "virus eaters" – by analogy with bacteriophages, viruses that parasitise bacteria – but the distinction between virophages and other satellite viruses that infect plants, arthropods and mammals is disputed. Other virophages have since been discovered, including the Mavirus, Zamilon and Organic Lake virophages; all infect protists and all rely on nucleocytoplasmic large DNA viruses as helpers. They have been proposed to belong to a new family, Lavidaviridae.

Sputnik's non-enveloped icosahedral capsid is 50 nm in diameter, and contains a circular double-stranded DNA genome of 18.3 kb. Three of its 21 predicted proteins are thought to derive from mamavirus or the related mimivirus, suggesting that virophages and giant viruses can swap genes during their joint infection of Acanthamoeba, and also that virophages might mediate horizontal gene transfer between giant viruses.

Did you know?

Jandokot Memorial to the "Sugarbird Lady", Robin Miller

Selected biography

Posthumous portrait of Randy Shilts

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.

In this month

Painting depicting Jenner inoculating Phipps by Ernest Board (c. 1910)

May 1955: First issue of Virology; first English-language journal dedicated to virology

4 May 1984: HTLV-III, later HIV, identified as the cause of AIDS by Robert Gallo and coworkers

5 May 1939: First electron micrographs of tobacco mosaic virus taken by Helmut Ruska and coworkers

5 May 1983: Structure of influenza neuraminidase solved by Jose Varghese, Graeme Laver and Peter Colman

8 May 1980: WHO announced formally the global eradication of smallpox

11 May 1978: SV40 sequenced by Walter Fiers and coworkers

12 May 1972: Gene for bacteriophage MS2 coat protein is sequenced by Walter Fiers and coworkers, the first gene to be completely sequenced

13 May 2011: Boceprevir approved for the treatment of chronic hepatitis C virus (HCV) infection, the first direct-acting antiviral for HCV

14 May 1796: Edward Jenner inoculated James Phipps (pictured) with cowpox

15/16 May 1969: Death of Robert Rayford, the earliest confirmed case of AIDS outside Africa

18 May 1998: First World AIDS Vaccine Day

20 May 1983: Isolation of the retrovirus LAV, later HIV, by Luc Montagnier, Françoise Barré-Sinoussi and coworkers

23 May 2011: Telaprevir approved for the treatment of chronic HCV infection

25 May 2011: WHO declared rinderpest eradicated

31 May 1937: First results in humans from the 17D vaccine for yellow fever published by Max Theiler and Hugh H. Smith

Selected intervention

The MMR vaccine and autism fraud refers to the false claim that the combined vaccine for measles, mumps and rubella (MMR) might be associated with colitis and autism spectrum disorders. Multiple large epidemiological studies have since found no link between the vaccine and autism. The notion originated in a fraudulent research paper by Andrew Wakefield and co-authors, published in the prestigious medical journal The Lancet in 1998. Sunday Times journalist Brian Deer's investigations revealed that Wakefield had manipulated evidence and had multiple undeclared conflicts of interest. The paper was retracted in 2010, when the Lancet's editor-in-chief Richard Horton characterised it as "utterly false". Wakefield was found guilty of serious professional misconduct by the General Medical Council, and struck off the UK's Medical Register. The claims in Wakefield's article were widely reported in the press, resulting in a sharp drop in vaccination uptake in the UK and Ireland. A greatly increased incidence of measles and mumps followed, leading to deaths and serious permanent injuries.

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