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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".

Selected disease

"Episode of Yellow Fever" by Juan Manuel Blanes (1871)

Yellow fever is an acute haemorrhagic fever caused by the yellow fever virus, an RNA virus in the Flaviviridae family. It infects humans, other primates, and Aedes aegypti and other mosquito species, which act as the vector. After transmission by the bite of a female mosquito, the virus replicates in lymph nodes, infecting dendritic cells, and can then spread to liver hepatocytes. Symptoms generally last 3-4 days, and include fever, nausea and muscle pain. In around 15% of people, a toxic phase follows with recurring fever, liver damage and jaundice, sometimes accompanied by bleeding and kidney failure; death occurs in 20-50% of those who develop jaundice. Infection otherwise leads to lifelong immunity.

The first definitive outbreak of yellow fever was in Barbados in 1647, and major epidemics have occurred in the Americas and southern Europe since that date. Yellow fever is endemic in tropical and subtropical areas of South America and Africa; its incidence has been increasing since the 1980s. An estimated 200,000 cases and 30,000 deaths occur each year, with almost 90% of cases being in Africa. Antiviral therapy is not effective. A vaccine is available, and vaccination, mosquito control and bite prevention are the main preventive measures.

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Aedes aegypti mosquito biting a human

Aedes aegypti can transmit the chikungunya, dengue, yellow fever and Zika viruses. The mosquito is widespread in tropical and subtropical regions, with mosquito control being key to disease prevention.

Credit: United States Department of Agriculture (2000)

Selected article

Cases of poliomyelitis in 2005 (top) and 2018 (bottom). Red: endemic; orange: re-established; green: imported; blue: vaccine derived; grey: none
Cases of poliomyelitis in 2005 (top) and 2018 (bottom). Red: endemic; orange: re-established; green: imported; blue: vaccine derived; grey: none

A global drive to eradicate poliovirus started in 1988, when there were an estimated 350,000 cases of wild poliovirus infection globally. Two diseases, both caused by viruses, have been eradicated, smallpox in 1980 and rinderpest in 2011. Poliovirus only infects humans. It persists in the environment for a few weeks at room temperature and a few months at 0-8 °C. The oral polio vaccine is inexpensive, highly effective and is predicted to generate lifelong immunity. Reversion of live vaccine strains to virulence has resulted in occasional cases of vaccine-associated paralysis.

Annual reported cases of wild poliovirus infection fell to a low of 22 in 2017, but rose to 33 in 2018. As of 2019, the wild virus remains endemic in three countries: Afghanistan, Pakistan and Nigeria. Vaccine-derived poliovirus is also circulating in several countries. A lack of basic health infrastructure and civil war remain significant obstacles to eradication. Some local communities have opposed immunisation campaigns, and vaccination workers have been murdered in Pakistan and Nigeria.

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

The masked palm civet (Paguma larvata) is thought to have been the source of SARS coronavirus

In the severe acute respiratory syndrome (SARS) outbreak, the first cases of the newly emerged SARS coronavirus were reported in November 2002 from the Chinese Guangdong province. The virus soon spread across Asia, with China, Hong Kong, Taiwan and Singapore being the worst affected countries; a secondary outbreak occurred in Canada. Over 8,000 people were infected, with nearly 10% dying. Those over 50 years had a much higher mortality rate, approaching half. The outbreak was contained by July 2003.

The immediate source of SARS coronavirus is likely to have been the masked palm civet (Paguma larvata), which was sold as food in Guangdong markets. The virus was also found in raccoon dogs, ferret badgers and domestic cats, and closely related coronaviruses have been isolated from bats, which probably form the natural reservoir. The rapid initial spread of the outbreak has been in part attributed to China's slow response to the early cases.

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Selected virus

Type 3 poliovirus capsid

Poliovirus is an enterovirus, an RNA virus in the Picornaviridae family, associated with the paralytic disease polio. The icosahedral virus particle is about 30 nanometres in diameter and lacks an envelope. It contains a relatively short, single-stranded positive RNA genome of around 7500 nucleotides, which encodes about ten viral products. The virus has a fairly high mutation rate even for an RNA virus. There are three serotypes, each with a slightly different capsid protein; PV1 is the most common.

The virus only naturally infects humans, although some monkeys can be infected experimentally; 95% of infections are asymptomatic. Infection occurs via the faecal-oral route and viral replication occurs in the alimentary tract. The virus enters the host cell by binding to an immunoglobulin-like receptor, CD155. Fully assembled poliovirus leaves the cell 4-6 hours after initiation of infection. Poliovirus was first isolated in 1909 by Karl Landsteiner and Erwin Popper. Its genome was sequenced in 1981. Among the simplest clinically significant viruses, poliovirus is one of the best-characterised viruses, and has become a useful model for studying RNA viruses.

Did you know?

Electron micrograph of Bourbon virus

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 2019, he directs the London School of Hygiene & Tropical Medicine.

In this month

Electron micrograph of SARS coronaviruses

7 November 1991: Magic Johnson announced his retirement from basketball because of his infection with HIV

14 November 1957: Kuru, the first human prion disease, described by Daniel Gajdusek and Vincent Zigas

16 November 2002: The first case of severe acute respiratory syndrome (virus pictured) recorded in Guangdong, China

17 November 1995: Lamivudine approved for treatment of HIV

22 November 2013: Simeprevir approved for treatment of chronic hepatitis C virus infection

23 November 1978: Structure of tomato bushy stunt virus solved by Stephen Harrison and colleagues, the first atomic-level structure of a virus

24 November 2007: Outbreak of new Ebola species, Bundibugyo virus

26 November 1898: Martinus Beijerinck coined the term contagium vivum fluidum to describe the agent causing tobacco mosaic disease

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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