Portal:Viruses

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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

Child with a measles rash

Measles is a disease that only affects humans caused by the measles virus, an RNA virus in the Paramyxoviridae family. It is highly contagious, with transmission occurring via the respiratory route or by contact with secretions. Symptoms generally develop 10–12 days after exposure and last 7–10 days; they include high fever, cough, rhinitis and conjunctivitis, white Koplik's spots inside the mouth and a generalised red maculopapular rash. Complications including diarrhoea, otitis media and pneumonia are relatively common; more rarely seizures, encephalitis, croup, corneal ulceration and blindness can occur. The risk of death is usually around 0.2%, but may be as high as 10–28% in areas with high levels of malnutrition and poor healthcare.

Measles was first described by Rhazes (860–932). The disease is estimated to have killed around 200 million people between 1855 and 2005. It affects about 20 million people a year, primarily in the developing areas of Africa and Asia, and is one of the leading vaccine-preventable disease causes of death. No antiviral drug is licensed. An effective measles vaccine is available, but uptake has been reduced by anti-vaccination campaigns, particularly the fraudulent claim that the MMR vaccine might be associated with autism. Rates of disease and deaths increased from 2017 to 2019, attributed to a decrease in immunisation.

Selected image

Ribbon diagram showing T7 RNA polymerase (blue) making messenger RNA (green) based on a DNA template (orange)

Some viruses, such as the T7 bacteriophage, encode their own RNA polymerase, the enzyme that makes messenger RNA based on a DNA template. The T7 enzyme has a single subunit, and is more like chloroplast and mitochondrial enzymes than those of bacteria or the cell.

Credit: Thomas Splettstoesser (25 June 2007)

In the news

Map showing the distribution of coronavirus cases; black: highest incidence; dark red to pink: decreasing incidence; grey: no recorded cases
Map showing the distribution of coronavirus cases; black: highest incidence; dark red to pink: decreasing incidence; grey: no recorded cases

4 April: The ongoing pandemic of a novel coronavirus is accelerating rapidly; more than a million confirmed cases, including more than 57,000 deaths, have been documented globally since the outbreak began in December 2019. WHO 1, 2

27 March: An international, randomised, non-blinded, clinical trial organised by the World Health Organization of four potential treatments for COVID-19remdesivir; chloroquine or hydroxychloroquine; lopinavir/ritonavir; or lopinavir/ritonavir plus interferon-beta – is about to start enrolling patients. Science, WHO

16 March: A phase I clinical trial of a messenger RNA-based vaccine candidate for the novel coronavirus begins in Seattle. NIH

11 March: The World Health Organization describes the ongoing outbreak of respiratory disease caused by a novel coronavirus as a pandemic. WHO

10 March: A patient with apparent clearance of HIV after stem-cell therapy continues to have no viable virus detectable in blood or other reservoirs after 30 months without antiretroviral treatment. Lancet

9 March: No new cases have been recorded in three weeks in the ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo; as of 3 March there had been a total of 3444 cases, including 2264 deaths, since the outbreak began in August 2018. WHO 1, 2

False-coloured electron micrograph of novel coronavirus
False-coloured electron micrograph of novel coronavirus

12 February: The ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo remains a Public Health Emergency of International Concern, according to the World Health Organization. WHO 1

7 February: Chinese scientists announce that novel coronavirus (2019-nCoV) is 99% identical to a coronavirus isolated from pangolins, suggesting these animals might be an intermediate host. Nature

5 February: A study of 2658 samples from 38 different types of cancer found that 16% were associated with a virus, higher than previous estimates, but did not identify any new candidate tumour viruses. Nat Genet

4 February: Over 2500 putative circular DNA virus genomes are catalogued from metagenomic surveys of human and animal samples, including over 600 dissimilar to existing virus groups. eLife, Science

3 February: The US National Institute of Allergy and Infectious Diseases stops the South African HVTN 702 Phase IIb/III clinical trial of an investigational HIV vaccine early, after the vaccine failed to prevent HIV infection. NIH

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.

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 and northern England, southern Scotland and Wales, with Cumbria being the worst-affected area. A small outbreak occurred in the Netherlands, and there were a few cases elsewhere in Europe.

The UK outbreak was controlled by the beginning of October. Control measures included stopping livestock movement and slaughtering over 6 million cows and sheep. Public access to farmland and moorland was also restricted (pictured), 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 £8 billion in the UK.

Selected quotation

Donald McNeil on the campaign to eradicate polio

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Selected virus

X-ray crystallographic structure of the bovine papillomavirus capsid

Papillomaviruses are small non-enveloped DNA viruses that make up the Papillomaviridae family. Their circular double-stranded genome is around 8000 nucleotides long. The icosahedral capsid is 55–60 nm in diameter. They infect humans, other mammals and some other vertebrates including birds, snakes, turtles and fish. Around a hundred species are classified into 53 genera. All papillomaviruses replicate exclusively in epithelial cells of stratified squamous epithelium, which forms the skin and some mucosal surfaces, including the lining of the mouth, airways, genitals and anus.

Infection by most papillomaviruses is either asymptomatic or causes small benign tumours known as warts or papillomas. Francis Peyton Rous showed in 1935 that the Shope papilloma virus could cause skin cancer in rabbits – the first time that a virus was shown to cause cancer in mammals – and papillomas caused by some virus types, including human papillomavirus 16 and 18, carry a risk of becoming cancerous if the infection persists. Papillomaviruses are associated with cancers of the cervix, vulva, vagina, penis, oropharynx and anus in humans.

Did you know?

Peanut plant (Arachis hypogaea)

Selected biography

Peter Piot in 2006

Peter Piot (born 17 February 1949) is a Belgian virologist and public health specialist, known for his work on Ebola virus and HIV.

During the first outbreak of Ebola in Yambuku, Zaire in 1976, Piot was one of a team that discovered the filovirus in a blood sample. He and his colleagues travelled to Zaire to help to control the outbreak, and showed that the virus is transmitted via blood and during preparation of bodies for burial. He advised WHO during the West African Ebola epidemic of 2014–16.

In the 1980s, Piot participated in collaborative projects in Burundi, Côte d'Ivoire, Kenya, Tanzania and Zaire, including Project SIDA in Kinshasa, the first international project on AIDS in Africa, which provided the foundations for understanding HIV infection in that continent. He was the founding director of UNAIDS, and has served as president of the International AIDS Society and assistant director of the WHO Global HIV/AIDS Programme. As of 2020, he directs the London School of Hygiene & Tropical Medicine.

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

25 October 2012: Alipogene tiparvovec, a gene therapy for lipoprotein lipase deficiency using an adeno-associated virus-based vector, was the first gene therapy to be licensed

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

27 October 2015: Talimogene laherparepvec was the first oncolytic virus to be approved by the FDA to treat cancer

Selected intervention

Child receiving the oral polio vaccine

Two polio vaccines are used against the paralytic disease polio. The first, developed by Jonas Salk, consists of inactivated poliovirus. Based on three wild virulent strains, inactivated using formalin, it is administered by injection and is very safe. It confers IgG-mediated immunity, which prevents poliovirus from entering the bloodstream and protects the motor neurons, eliminating the risk of bulbar polio and post-polio syndrome. The second, developed by Albert Sabin, originally consisted of three live virus strains, attenuated by growth in cell culture. Since 2016, only two strains have generally been included. They contain multiple mutations, preventing them from replicating in the nervous system. The Sabin vaccine stimulates both antibodies and cell-mediated immunity, providing longer-lasting immunity than the Salk vaccine. It can be administered orally, making it more suitable for mass vaccination campaigns. In around three cases per million doses, the live vaccine reverts to a virulent form and causes paralysis. Vaccination has reduced the number of wild-type polio cases from around 350,000 in 1988 to just 33 in 2018, and eradicated the disease from most countries.

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