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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".

Selected disease

Chickenpox rash in an adult male

Chickenpox is caused by initial infection with varicella zoster virus, a DNA virus of the Alphaherpesvirinae subfamily. The virus naturally infects only humans, but some other primates have been infected artificially. Symptoms appear 10–21 days after exposure: an itchy vesicular skin rash, and small ulcers in the oral cavity and tonsil areas. The rash usually resolves by 7 days, but the virus remains latent in nerve cell bodies, and can emerge years or decades later to cause shingles. Chickenpox is transmitted by the respiratory route, as well as direct contact with lesions.

A classic disease of childhood, the highest prevalence occurs at 4–10 years. Chickenpox is rarely fatal in people with a normal immune system, with around 6,400 deaths worldwide in 2015, about 1 in 60,000 infections. Adults often have more severe symptoms than children, and are at higher risk of complications such as pneumonia, bronchitis, hepatitis and encephalitis. Pregnant women and people with a suppressed immune system have the highest complication risk. Chickenpox during the first 28 weeks of gestation can lead to foetal malformations. Infection in adults is usually treated with antiviral drugs, such as aciclovir or valaciclovir, which reduces symptom severity and the risk of complications. A vaccine is available.

Selected image

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)

Selected article

Child receiving oral polio vaccine in India

Vaccination or immunisation is the administration of immunogenic material (a vaccine) to stimulate an individual's immune system to develop adaptive immunity to a virus or other pathogen, and so develop protection against an infectious disease. The active agent of a vaccine may be intact but inactivated or weakened forms of the pathogen, or purified highly immunogenic components, 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. When a sufficiently high proportion of a population has been vaccinated, herd immunity results. Widespread immunity due to mass vaccination is largely responsible for the worldwide eradication of smallpox and the elimination of diseases such as polio 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.

In the news

Diagram of African swine fever virus

2 July: In the ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo (DRC) – now the second-largest Ebola outbreak in history – new cases remain stable, with 285 reported in the past 3 weeks, and a total of 2372 cases, including 1602 deaths, since the outbreak began in August 2018. WHO 1, 2

26 June: A meta-analysis of vaccinating girls and women against human papillomavirus including 60 million vaccinees finds that diagnoses of high-grade cervical intraepithelial neoplasia and cervical cancer in women and of anogenital warts in both women and men have reduced in frequency. Lancet

14 June: The WHO Emergency Committee declares that the ongoing DRC Ebola virus outbreak fails to meet the criteria for a Public Health Emergency of International Concern, despite the spread to Uganda. 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

Selected outbreak

Notice prohibiting access to the North Yorkshire moors during the outbreak

The 2001 foot-and-mouth outbreak included 2,000 cases of the disease in cattle and sheep across the UK. The source was a Northumberland farm where pigs had been fed infected meat that had not been adequately sterilised. The initial cases were reported in February. The disease was concentrated in western England, southern Scotland and Wales, with Cumbria being the worst-affected area. A small outbreak occurred in the Netherlands, and there were a handful of cases elsewhere in Europe.

The UK outbreak was controlled by the beginning of October. Control measures included stopping animal movement and slaughtering over 10 million cows and sheep. Access to farmland and moorland was also restricted, greatly reducing tourism in affected areas, particularly in the Lake District. Vaccination was used in the Netherlands, but not in the UK due to concerns that vaccinated livestock could not be exported. The outbreak cost an estimated £8bn in the UK.

Selected quotation

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Selected virus

False-coloured electron micrograph of Hendra virus

Henipaviruses are a genus of RNA viruses in the Paramyxoviridae family. The variably shaped, 40–600 nm diameter, enveloped capsid contains a single-stranded, negative-sense RNA genome of 18.2 kb, encoding six proteins. The cellular receptor Ephrin-B2 is widely distributed in mammals. Their natural hosts are bats, mainly megabats (fruit bats or flying foxes) and some microbats. Bats infected with Hendra virus develop viraemia and shed virus in urine, faeces and saliva for around a week, but show no signs of disease. Henipaviruses can also infect humans and livestock, causing severe disease with high mortality, making the group a zoonootic disease.

The first henipavirus, Hendra virus, was discovered in 1994 as the cause of an outbreak in horses in Brisbane, Australia. Two other species are known, Nipah and Cedar viruses. Their emergence as human pathogens has been linked to increased contact between bats and humans, sometimes via an intermediate domestic animal host. Human disease has been confined to Australia and Asia, but related sequences have also been found in African bats. Only a veterinary vaccine against Hendra virus is available.

Did you know?

Tripneustes depressus

Selected biography

Ali Maow Maalin (1954 – 22 July 2013) was a hospital cook and health worker from Merca, Somalia, who is the last person in the world known to be infected with naturally occurring smallpox. Although he worked in the local smallpox eradication programme, he had not been successfully vaccinated. In October 1977, he was infected with the Variola minor strain of the virus while driving two children with smallpox symptoms to quarantine. He did not experience complications and made a full recovery. An aggressive containment campaign was successful in preventing an outbreak, and smallpox was declared to have been eradicated globally by the World Health Organization (WHO) two years later.

In later life, Maalin volunteered for the successful poliomyelitis eradication campaign in Somalia. He worked for WHO as a local coordinator with responsibility for social mobilisation, and spent several years travelling across Somalia, vaccinating children and educating communities. He encouraged people to be vaccinated by sharing his experiences with smallpox. He died of malaria while carrying out polio vaccinations after the reintroduction of poliovirus to the country in 2013.

In this month

Louis Pasteur in 1878

1 July 1796: Edward Jenner first challenged James Phipps with variolation, showing that cowpox inoculation is protective against smallpox

3 July 1980: Structure of southern bean mosaic virus solved by Michael Rossmann and colleagues

6 July 1885: Louis Pasteur (pictured) gave rabies vaccine to Joseph Meister

10 July 1797: Jenner submitted paper on Phipps and other cases to the Royal Society; it was read to the society but not published

14–20 July 1968: First International Congress for Virology held in Helsinki

16 July 2012: FDA approved tenofovir/emtricitabine (Truvada) for prophylactic use against HIV; first prophylactic antiretroviral

19 July 2013: Pandoravirus described, with a genome twice as large as Megavirus

22 July 1966: International Committee on Nomenclature of Viruses (later the ICTV) founded

25 July 1985: Film star Rock Hudson made his AIDS diagnosis public, increasing public awareness of the disease

28 July 2010: First global World Hepatitis Day

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.

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