Portal:Viruses

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The Viruses Portal
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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 6,000 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

Foot-and-mouth disease or FMD is an economically important disease of even-toed ungulates (cloven-hoofed animals) 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 extremely rare. After a 1–12-day incubation, animals develop high fever, and then blisters inside the mouth (pictured) and on the hooves, which can rupture and cause lameness. Weight loss and reduction in milk production are other possible long-term consequences. Mortality in adult animals is low (2–5%). The virus is highly infectious, with transmission occurring via direct contact, aerosols, semen, consumption of infected food scraps or feed supplements, and via inanimate objects including fodder, farming equipment, vehicles, standing water, and the clothes and skin of humans. Some infected ruminants can transmit infection as asymptomatic carriers.

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. The virus is highly variable, with seven serotypes. A vaccine is available, but protection is temporary and strain specific. Other control methods include monitoring programmes, trade restrictions, quarantine, and the slaughter of infected and healthy at-risk animals.

Selected image

Biosafety level 3 equipment is used for research with viruses such as influenza that can cause serious disease but for which treatment is available. The biosafety cabinet uses HEPA filters to filter viruses out of the air. This researcher is examining reconstructed 1918 pandemic influenza virus, or "Spanish flu".

Credit: CDC (2005)

In the news

Map showing the prevalence of SARS-CoV-2 cases; black: highest prevalence; dark red to pink: decreasing prevalence; grey: no recorded cases or no data

26 February: In the ongoing pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), more than 110 million confirmed cases, including 2.5 million deaths, have been documented globally since the outbreak began in December 2019. WHO

18 February: Seven asymptomatic cases of avian influenza A subtype H5N8, the first documented H5N8 cases in humans, are reported in Astrakhan Oblast, Russia, after more than 100,0000 hens died on a poultry farm in December. WHO

14 February: Seven cases of Ebola virus disease are reported in Gouécké, south-east Guinea. WHO

7 February: A case of Ebola virus disease is detected in North Kivu Province of the Democratic Republic of the Congo. WHO

4 February: An outbreak of Rift Valley fever is ongoing in Kenya, with 32 human cases, including 11 deaths, since the outbreak started in November. WHO

21 November: The US Food and Drug Administration (FDA) gives emergency-use authorisation to casirivimab/imdevimab, a combination monoclonal antibody (mAb) therapy for non-hospitalised people twelve years and over with mild-to-moderate COVID-19, after granting emergency-use authorisation to the single mAb bamlanivimab earlier in the month. FDA 1, 2

18 November: The outbreak of Ebola virus disease in Équateur Province, Democratic Republic of the Congo, which started in June, has been declared over; a total of 130 cases were recorded, with 55 deaths. UN

Selected article

16th-century Aztec print showing a person with measles

Viruses and viral infections have affected human history. Epidemics caused by viruses began when human behaviour changed during the Neolithic period, around 12,000 years ago. Previously hunter-gatherers, humans developed more densely populated agricultural communities, which allowed viruses to spread rapidly and subsequently to become endemic. Viruses of plants and livestock also increased, and as humans became dependent on agriculture and farming, diseases such as potyviruses of potatoes and rinderpest of cattle had devastating consequences. Smallpox and measles viruses are among the oldest that infect humans. They first appeared in humans in Europe and North Africa thousands of years ago, having evolved from viruses that infected other animals. Influenza pandemics have been recorded since 1580.

There are an estimated 10³¹ viruses on Earth. Although scientific interest in them arose because of the diseases they cause, most viruses are beneficial. They drive evolution by transferring genes across species, play important roles in ecosystems, and are essential to life.

Selected outbreak

The 1993 hantavirus outbreak in the Four Corners region of southwest USA was of a novel hantavirus, subsequently named Sin Nombre virus. It caused the previously unrecognised hantavirus pulmonary syndrome – the first time that a hantavirus had been associated with respiratory symptoms. Mild flu-like symptoms were followed by the sudden onset of pulmonary oedema, which was fatal in half of those affected. A total of 24 cases were reported in April–May 1993, with many of those affected being from the Navajo Nation territory. Hantavirus infection of humans generally occurs by inhaling aerosolised urine and faeces of rodents, in this case the deer mouse (Peromyscus; pictured).

Previously documented hantavirus disease had been confined to Asia and Europe, and these were the first human cases to be recognised in the USA. Subsequent investigation revealed undiagnosed cases dating back to 1959, and Navajo people recalled similar outbreaks in 1918, 1933 and 1934.

Selected quotation

“ The unusual features of the giant Mimivirus revived the popular, yet unresolved question: "Are viruses alive?" The discovery that some of them can get sick adds a new twist to this old debate. ”

—Hiroyuki Ogata & Jean-Michel Claverie on the relationship between Sputnik virophage and mimivirus

Recommended articles

Viruses & Subviral agents: bat virome • elephant endotheliotropic herpesvirus • HIV • introduction to viruses • Playa de Oro virus • poliovirus • prion • rotavirus • virus

Diseases: colony collapse disorder • common cold • croup • dengue fever • gastroenteritis • Guillain–Barré syndrome • hepatitis B • hepatitis C • hepatitis E • herpes simplex • HIV/AIDS • influenza • meningitis • myxomatosis • polio • pneumonia • shingles • smallpox

Epidemiology & Interventions: 2007 Bernard Matthews H5N1 outbreak • Coalition for Epidemic Preparedness Innovations • Disease X • 2009 flu pandemic • HIV/AIDS in Malawi • polio vaccine • Spanish flu • West African Ebola virus epidemic

Virus–Host interactions: antibody • host • immune system • parasitism • RNA interference

Methodology: metagenomics

Social & Media: And the Band Played On • Contagion • "Flu Season" • Frank's Cock • Race Against Time: Searching for Hope in AIDS-Ravaged Africa • social history of viruses • "Steve Burdick" • "The Time Is Now" • "What Lies Below"

People: Brownie Mary • Macfarlane Burnet • Bobbi Campbell • Aniru Conteh • people with hepatitis C • HIV-positive people • Bette Korber • Henrietta Lacks • Linda Laubenstein • Barbara McClintock • poliomyelitis survivors • Joseph Sonnabend • Eli Todd • Ryan White

Selected virus

Hantaviruses (or orthohantaviruses) are a family of RNA viruses in the Bunyavirales order. The enveloped virion is 120–160 nm in diameter and contains a single-stranded, negative-sense RNA genome with three segments. They infect many species of rodents, as well as shrews and moles, without causing disease, and can be transmitted to humans, where they can cause serious disease. Hantaan virus, the first known hantavirus, was discovered in 1976 as the cause of a novel haemorrhagic fever affecting combatants in the Korean War; it can also be caused by other hantaviruses, including Dobrava-Belgrade and Seoul viruses. Some hantaviruses, including Sin Nombre and Bayou, cause a pulmonary syndrome. Others have not yet been associated with human disease.

Unlike other bunyaviruses, hantaviruses are not transmitted by arthropods. Rodents act as the vector, with transmission to humans usually occurring via contact with urine, saliva or faeces, by inhalation of aerosolised excreta or by bite. Little is understood about how hantaviruses cause disease; the main site of viral replication in the body is unknown. Rodent control is important in disease prevention.

Did you know?

Harald zur Hausen, photographed by Armin Kübelbeck

• ...that virologist Harald zur Hausen received both the Gairdner Foundation International Award and the Nobel Prize in Physiology or Medicine in 2008?
• ...that in 2007, the first Australian outbreak of equine influenza caused a nationwide ban on horse racing?
• ...that Michael S. Gottlieb was instrumental in identifying AIDS in 1981 and thereafter treated actor Rock Hudson for the disease?
• ...that the tick Ornithodoros erraticus spreads the African swine fever virus in the Iberian Peninsula (Spain and Portugal)?
• ...that the CEO of CEPI, a key organisation in the race to develop a vaccine for COVID-19, has called the disease "the most frightening" that he has ever encountered in his career?

Selected biography

Ryan Wayne White (6 December 1971 – 8 April 1990) was an HIV-positive American teenager who became a national spokesman for AIDS research and public education about HIV/AIDS, after being expelled from school because of his infection.

White, a haemophiliac, was diagnosed in 1984 after infection by a contaminated blood treatment. HIV/AIDS was then poorly understood, and his return to school in Kokomo, Indiana was prevented by protesters; the ensuing legal battle gained national media coverage. Before his case, AIDS was widely associated with the male gay community; White was one of several who helped to shift that perception.

White died in 1990, one month before his high school graduation. Shortly afterwards, the U.S. Congress passed a major piece of AIDS legislation, the Ryan White Care Act. Ryan White Programs remain the largest provider of services for people living with HIV/AIDS in the United States.

In this month

5 June 1981: First report of HIV/AIDS (symbol pictured) appeared in medical literature

6 June 1997: Gene silencing in plants shown to be a viral defence mechanism

7–13 June 1962: Donald Caspar and Aaron Klug proposed the quasi-equivalence principle of virus structure

7–13 June 1962: André Lwoff proposed a viral classification scheme based on nature of genome, type of symmetry and presence of envelope

7–13 June 1962: George Hirst proposed that the influenza virus genome is segmented

9 June 1981: The American Society for Virology was founded

13 June 2012: First case of Middle East respiratory syndrome coronavirus (MERS-CoV) occurred in Saudi Arabia

18 June 1981: A vaccine against foot-and-mouth disease was the first genetically engineered vaccine

21 June 1996: Nevirapine approved, first NNRTI for HIV/AIDS

26 June 1993: Clinical trial of hepatitis B virus drug fialuridine terminated; the drug caused several fatalities due to lactic acidosis

28 June 2011: FAO declared rinderpest eradicated

30 June 1985: Ryan White was denied re-admittance to his school after an AIDS diagnosis, in a case that changed public perceptions of the disease

More events

Selected intervention

Oseltamivir (also Tamiflu) is an oral antiviral drug against influenza (flu). It was the second inhibitor of the viral neuraminidase to be developed, after zanamivir, and the first to be taken as an oral tablet. It was originally synthesised from shikimic acid extracted from the star anise plant. Oseltamivir is a prodrug that requires metabolism in the liver to the active form, oseltamivir carboxylate. This binds at the active site of the neuraminidase enzyme, preventing it from cleaving sialic acid to release the virus particle from the host cell. Oseltamivir can reduce the duration of influenza symptoms by 0.5–1 days. Debate is ongoing about whether it also reduces the risk of complications, such as pneumonia. Nausea and vomiting are the main adverse events. Resistance to oseltamivir has been observed in some strains of influenza virus, especially H1N1 strains.

Subcategories

Subcategories of virology:

Virology

Branches of virology

Viruses

Diagnostic virology

Viral diseases

Viral genes

History of virology

Virology journals

Viral life cycle

Virology organizations

Viral proteins

Satellite viruses

Slow virus diseases

Subviral agents

Viroids

Virologists

Virotherapy

Virucides

Viruses

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