The Viruses Portal
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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
Hepatitis C is an infectious disease affecting mainly the liver, caused by hepatitis C virus (HCV), an RNA virus of the Flaviviridae family which only infects humans and chimpanzees. A "non-A non-B hepatitis" was postulated in the 1970s, and HCV was demonstrated in 1989. HCV is spread primarily by blood-to-blood contact associated with intravenous drug use in the developed world, and with improperly sterilised medical equipment and blood transfusions in the developing world. In about 80% of those infected, the virus establishes a chronic infection in the liver, and around 10–30% of those infected will develop cirrhosis over 30 years. Some people with cirrhosis go on to develop liver failure, liver cancer or other serious complications.
An estimated 143 million people worldwide (2%) have chronic HCV infections as of 2015. The prevalence is highest in Central and East Asia, North Africa and the Middle East. The virus causes around a quarter of cases of cirrhosis and hepatocellular carcinoma, and is a major reason for liver transplantation. The recommended therapy is an appropriate combination of protease inhibitors. Although 95% of people treated in this way are cured, the treatments are expensive and older therapies are less effective. No vaccine against hepatitis C is available.
Selected image
The antiviral fomivirsen was the first antisense therapy to be licensed by the FDA. It binds to a cytomegalovirus mRNA and is used to treat cytomegalovirus retinitis.
Credit: Fvasconcellos (1 January 2007)
In the news
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
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The Hershey–Chase experiments were conducted by Alfred Hershey and Martha Chase in 1952 using the T2 bacteriophage (pictured), which is composed of DNA wrapped in a protein shell. Hershey and Chase labelled either the phage DNA using radioactive phosphorus-32 or the protein using radioactive sulphur-35. They allowed the radiolabelled phages to infect unlabelled bacteria, and then agitated in a blender and centrifuged to separate material remaining outside the bacterial cells. The majority of the 32P-labelled DNA entered the host bacterial cell, while all the 35S-labelled protein remained outside. Hershey and Chase also showed that the phage DNA is inserted into the bacteria shortly after the virus attaches to its host.
DNA had been known since 1869, but in 1952 many scientists believed that proteins carried the information for inheritance. Proteins appeared more complex, while DNA was thought to be an inert molecule used for phosphorus storage. These experiments built on earlier research on transformation in bacteria and helped to confirm that DNA, not protein, was the genetic material. Hershey shared the 1969 Nobel Prize in Physiology or Medicine for the research.
Selected outbreak
The last recorded smallpox death occurred during the 1978 smallpox outbreak in Birmingham, UK. The outbreak resulted from accidental exposure to the Abid strain of Variola major, from a laboratory, headed by Henry Bedson, at the University of Birmingham Medical School – also associated with an outbreak in 1966. Bedson was investigating strains of smallpox known as whitepox, considered a potential threat to the smallpox eradication campaign, then in its final stages.
A medical photographer who worked on the floor above the laboratory showed smallpox symptoms in August and died the following month; one of her contacts was also infected but survived. The government inquiry into the outbreak concluded that she had been infected in late July, possibly via ducting, although the precise route of transmission was subsequently challenged. The inquiry criticised the university's safety procedures. Bedson committed suicide while under quarantine. Radical changes in UK research practices for handling dangerous pathogens followed, and all known stocks of smallpox virus were concentrated in two laboratories.
Selected quotation
| “ | ...getting rid of the last 1 percent has been like trying to squeeze Jell-O to death. As the vaccination fist closes in one country, the virus bursts out in another... | ” |
—Donald McNeil on the campaign to eradicate polio
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
Epstein–Barr virus (EBV) (also human herpesvirus 4) is a DNA virus in the Herpesviridae family which infects humans. The virion is around 122–180 nm in diameter. As in all herpesviruses, the nucleocapsid is surrounded by a protein tegument, as well as an envelope. The double-stranded DNA genome is about 172 kb, with 85 genes, making it one of the more complex viruses.
Transmission is via saliva and genital secretions. The virus infects epithelial cells in the pharynx and B cells of the immune system, producing virions by budding. EBV also becomes latent in B cells, possibly in the bone marrow, allowing the infection to persist lifelong. In the latent state, the linear genome is made circular and replicates in the nucleus separately from the host DNA as an episome. Reactivation is poorly understood but is thought to be triggered by the B cell responding to other infections. EBV infection occurs in around 95% of people. Infectious mononucleosis or glandular fever can occur when first infection is delayed until adolescence or adulthood. EBV is associated with some types of cancer, including Burkitt's lymphoma and nasopharyngeal carcinoma. In people with HIV, it can cause hairy leukoplakia and central nervous system lymphomas.
Did you know?
- ...that Australian pilot and nurse Robin Miller (monument pictured) borrowed money to buy a Cessna 182 and flew to remote outback areas in Western Australia to vaccinate Indigenous Australian children against polio, becoming known as the "Sugarbird Lady"?
- ...that the Mokola virus is a relative of the rabies virus and was first isolated in tree shrews?
- ...that in 1995–97, a Centers for Disease Control program identified over a hundred patients likely suffering from life-threatening infections new in the history of emerging infectious diseases?
- ...that Batheay District in Kampong Cham Province, Cambodia, was the site of an outbreak of the deadly H5N1 virus in 2006?
- ...that Fig Trees, an operatic documentary about AIDS activism, is narrated by a singing albino squirrel?
Selected biography
George Keble Hirst (2 March 1909 – 22 January 1994) was an American virologist who was among the first to study the molecular biology and genetics of animal viruses.
Hirst started to work on influenza virus in 1940, only a few years after it had been isolated. He soon discovered that the virus caused red blood cells to clump together. This phenomenon could be used to diagnose influenza, which had previously required growing the virus in ferrets. He invented the haemagglutination assay, a simple method for quantifying viruses, and later the haemagglutination inhibition assay, which measures virus-specific antibodies in serum. In 1942, he discovered the neuraminadase enzyme, showing for the first time that viruses could contain enzymes. Neuraminidase is the target of the neuraminidase inhibitor class of antiviral drugs, including oseltamivir and zanamivir. In 1962, he was also the first to propose the then-revolutionary idea that virus genomes can consist of discontinuous segments.
He co-founded Virology in 1955, the first English-language journal to focus on viruses, and directed the Public Health Research Institute in New York City for nearly 25 years (1956–81).
In this month
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
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.
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