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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

Cow with a ruptured blister in the mouth, a sign of foot-and-mouth disease

Foot-and-mouth disease or FMD is an economically important disease of even-toed ungulates and some other mammals caused by the FMD virus, a picornavirus. Hosts include cattle, water buffalo, sheep, goats, pigs, antelope, deer and bison; human infection is very rare. After a 1–12-day incubation, animals develop fever, and then blisters on the hooves and inside the mouth. Mortality in adult animals is low (2–5%). Long-term consequences can include lameness, weight loss and reduction in milk production. The virus is highly infectious, with transmission occurring via direct contact, aerosols, semen, consumption of infected meat scraps or feed supplements, and via inanimate objects including fodder, farming equipment, vehicles and the clothes and skin of humans. Asymptomatic carriers can transmit infection.

Friedrich Loeffler showed the disease to be viral in 1897. FMD was widely distributed in 1945. By 2014, North America, Australia, New Zealand, much of Europe, and some South American countries were free of the disease. Major outbreaks include one in the UK in 2001 that cost an estimated £8 billion. A vaccine is available, but protection is temporary and strain specific. Other methods of control include monitoring programmes, export bans, quarantine and slaughter of at-risk animals.

Selected image

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)

Selected article

Culex mosquitoes are the vectors for West Nile Virus

Infectious diseases are symptomatic diseases resulting from the infection and replication of pathogens, including viruses, prions, bacteria, fungi, protozoa and multicellular parasites, in an individual host. Infectious diseases were responsible for over a quarter of human deaths globally in 2002, with HIV, measles and influenza being among the most significant viral causes of death.

Infectious pathogens must enter, survive and multiply within the host, and spread to fresh hosts. Relatively few microorganisms cause disease in healthy individuals, and infectious disease results from the interplay between these rare pathogens and the host's defences. Infection does not usually result in the host's death, and the pathogen is generally cleared from the body by the host's immune system. Transmission can occur by physical contact, contaminated food, body fluids, objects, airborne inhalation or via vectors, such as the mosquito (pictured). Diagnosis often involves identifying the pathogen; techniques include culture, microscopy, immunoassays and PCR-based molecular diagnostics.

In the news

False-coloured micrograph of Ebola virus particles (green) budding from infected cell (blue)

7 February: Horse racing across Britain is suspended after horses from a Cheshire stable test positive for equine influenza. BBC

5 February: In the ongoing Ebola virus outbreak (infected cell pictured) in the North Kivu and Ituri provinces of the Democratic Republic of the Congo, there have been 119 new cases in the past 3 weeks, with a total of 789 cases, including 488 deaths, since the outbreak began in August 2018; this is now the second-largest Ebola outbreak in history. WHO

21 January: In the ongoing outbreak of dengue fever in Jamaica, there have been 339 suspected and confirmed cases in the past 3 weeks, including 6 deaths. WHO

20 January: An outbreak of hantavirus pulmonary syndrome is ongoing in Epuyén, Chubut Province, southern Argentina, with 29 confirmed cases and 11 deaths. WHO

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.

Selected quotation

Félix d'Herelle on the discovery of bacteriophages

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Selected virus

Electron micrograph of West Nile virus

West Nile virus (WNV) is a flavivirus, an RNA virus in the Flaviviridae family. The enveloped virion is 45–50 nm in diameter and contains a single-stranded +RNA genome of 11–12 kb, encoding ten proteins. The main natural hosts are passerine and other birds (the reservoir), and several species of Culex mosquito (the vector); transmission is by bite of the female. WNV can also infect humans, some non-human primates, and some other mammals, including horses, as well as reptiles and amphibians. Mammals form a dead end for the virus, as it cannot replicate sufficiently quickly in them to complete the cycle back to the mosquito.

First identified in Uganda in 1937, WNV caused only sporadic human disease until the 1990s, but is now endemic in Africa, Asia, Australia, the Middle East, Europe and the United States. A fifth of humans infected experience West Nile fever, a mild flu-like disease. In less than 1% of those infected, the virus invades the central nervous system, with symptoms including encephalitis, meningitis and flaccid paralysis. No antiviral treatment is effective. No vaccine is currently available; mosquito control is the main preventive measure.

Did you know?

Jandokot Memorial to the "Sugarbird Lady", Robin Miller

Selected biography

Edward Jenner, by James Northcote (1800s)

Edward Jenner (17 May 1749 – 26 January 1823) was an English physician and scientist from Berkeley, Gloucestershire, who was the pioneer of the smallpox vaccine.

Noting the common observation that milkmaids were generally immune to smallpox, Jenner postulated that the pus in the blisters that milkmaids received from cowpox (a similar but much less virulent disease) protected them from smallpox. Jenner tested his hypothesis by inoculating an eight-year-old boy with such pus. He subsequently repeatedly challenged the boy with variolous material, then the standard method of immunisation, without inducing disease. Although others had previously inoculated subjects with cowpox, Jenner was the first to show that the procedure induced immunity to smallpox. He later successfully popularised cowpox vaccination.

Jenner is often called "the father of immunology", and his work is said to have "saved more lives than the work of any other man".

In this month

Dmitri Ivanovsky

February 1939: First virology journal, Archiv für die gesamte Virusforschung, appeared

8 February 1951: Establishment of the HeLa cell line from a cervical carcinoma biopsy, the first immortal human cell line

12 February 1892: Dmitri Ivanovsky (pictured) demonstrated transmission of tobacco mosaic disease by extracts filtered through Chamberland filters; sometimes considered the beginning of virology

19 February 1966: Prion disease kuru shown to be transmissible

23 February 2018: Baloxavir marboxil, the first anti-influenza agent to target the cap-dependent endonuclease activity of the viral polymerase, approved in Japan

27 February 2005: H1N1 influenza strain resistant to oseltamivir reported in a human patient

24 February 1977: Phi X 174 sequenced by Fred Sanger and coworkers, the first virus and the first DNA genome to be sequenced

28 February 1998: Publication of Andrew Wakefield's Lancet paper, subsequently discredited, linking the MMR vaccine with autism, which started the MMR vaccine controversy

Selected intervention

Ball-and-stick model of nevirapine

Nevirapine (also Viramune) is an antiretroviral drug used in the treatment of HIV/AIDS. It was the first non-nucleoside reverse transcriptase inhibitor to be licensed, which occurred in 1996. Like nucleoside inhibitors, nevirapine inhibits HIV's reverse transcriptase enzyme, which copies the viral RNA into DNA and is essential for its replication. However, unlike nucleoside inhibitors, it binds not in the enzyme's active site but in a nearby hydrophobic pocket, causing a conformational change in the enzyme that prevents it from functioning. Mutations in this pocket generate resistance to nevirapine, which develops rapidly unless viral replication is completely suppressed. The drug is therefore used together with other anti-HIV drugs in combination therapy called highly active antiretroviral therapy (HAART). A single dose of nevirapine is a cost-effective way to reduce mother-to-child transmission of HIV, and has been recommended by the World Health Organization for use in resource-poor settings. Rash is the most common adverse event associated with the drug.


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