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
Shingles, or herpes zoster, is a painful skin rash with blisters that, characteristically, occurs in a stripe limited to just one side of the body. The rash usually heals within 2–5 weeks, but around one in five people experience residual nerve pain for months or years.
Shingles is caused by varicella zoster virus (VZV), an alpha-herpesvirus. Initial VZV infection usually occurs in childhood causing chickenpox. After this resolves, the virus is not eliminated from the body, but remains latent in the nerve cell bodies of the dorsal root or trigeminal ganglia, without causing symptoms. Years or decades later, shingles occurs when virions in a single ganglion reactivate, travel down nerve fibres and infect the skin around the nerve. The shingles rash is restricted to the area of skin supplied by a single spinal nerve, termed the dermatome. Exactly how VZV remains latent in the body, and subsequently reactivates, is unclear.
Around a third of the population will develop shingles. Repeated episodes are rare. In the United States, about half the cases occur in people aged 50 years or older. Vaccination at least halves the risk, and prompt treatment with aciclovir or related antiviral drugs can reduce the severity and duration of the rash.
Selected image
Many small icosahedral viruses have capsids made up of multiple copies of just two proteins. The proteins aggregate into units called capsomeres, which have either pentagonal or hexagonal symmetry (as shown here).
Credit: Antares42 (4 September 2009)
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
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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 1976 Zaire Ebola virus outbreak was one of the first two recorded outbreaks of the disease. The causative agent was identified as a novel virus, named for the region's Ebola River. The first identified case, in August, worked in the school in Yambuku, a small rural village in Mongala District, north Zaire. He had been treated for suspected malaria at the Yambuku Mission Hospital, which is now thought to have spread the virus by giving vitamin injections with inadequately sterilised needles, particularly to women attending prenatal clinics. Unsafe burial practices also spread the virus.
The outbreak was contained by quarantining local villages, sterilising medical equipment and providing protective clothing to medical personnel, and was over by early November. A total of 318 cases was recorded, of whom 280 died, an 88% case fatality rate. An earlier outbreak in June–November in Nzara, Sudan, was initially thought to be linked, but was shown to have been caused by a different species of Ebola virus.
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| “ | Viruses are living chemicals... | ” |
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
Canine parvovirus type 2 (CPV2) is a non-enveloped, single-stranded DNA virus in the Parvoviridae family. The icosahedral viral capsid is only 20–26 nm in diameter, making it one of the smallest viruses. The genome is about 5000 nucleotides long. The virus is very similar to feline panleukopenia virus, another parvovirus, as well as mink enteritis and raccoon and fox parvoviruses. It infects dogs, wolves, foxes and other canids, big cats and occasionally domestic cats, but cannot infect humans.
A relatively new disease, CPV2 infection was first recognised in 1978 and rapidly spread worldwide. The virus is stable and highly infectious, being transmitted by contact with faeces, infected soil or contaminated objects. After ingestion, the virus replicates in the lymphoid tissue in the throat, then spreads to the bloodstream to infect cells of the lymph nodes, intestinal crypts and bone marrow, damaging the intestinal lining. The more common intestinal form of disease causes vomiting and severe, often bloody diarrhoea. The cardiac form affects puppies under 8 weeks, causing respiratory or cardiovascular failure; mortality can reach 91% in untreated cases. No specific antiviral drug is available. Prevention is by vaccination.
Did you know?
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- ...that a COVID-19 outbreak at the White House (event pictured) infected at least 35 people, including the President, First Lady, three senators, and a governor?
- ...that Alasdair Geddes worked on the WHO's smallpox-eradication programme in Bangladesh five years before diagnosing the world's last fatal case of smallpox in Birmingham, England?
- ...that an HPV prevention group focuses on preventing cervical cancer by promoting the sharing of information on cervical screening and HPV vaccination?
- ...that in a 1902 U.S. Supreme Court case, a ship owner alleged that the real purpose of Louisiana's quarantine laws was to keep Italian immigrants out of New Orleans?
- ...that Melahat Okuyan, a Turkish female microbiologist and AIDS activist, once proposed the establishment of male brothels for homosexuals and cross-dressers in order to improve public health?
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
May 1955: First issue of Virology; first English-language journal dedicated to virology
4 May 1984: HTLV-III, later HIV, identified as the cause of AIDS by Robert Gallo and coworkers
5 May 1939: First electron micrographs of tobacco mosaic virus taken by Helmut Ruska and coworkers
5 May 1983: Structure of influenza neuraminidase solved by Jose Varghese, Graeme Laver and Peter Colman
8 May 1980: WHO announced formally the global eradication of smallpox
11 May 1978: SV40 sequenced by Walter Fiers and coworkers
12 May 1972: Gene for bacteriophage MS2 coat protein is sequenced by Walter Fiers and coworkers, the first gene to be completely sequenced
13 May 2011: Boceprevir approved for the treatment of chronic hepatitis C virus (HCV) infection, the first direct-acting antiviral for HCV
14 May 1796: Edward Jenner inoculated James Phipps (pictured) with cowpox
15/16 May 1969: Death of Robert Rayford, the earliest confirmed case of AIDS outside Africa
18 May 1998: First World AIDS Vaccine Day
20 May 1983: Isolation of the retrovirus LAV, later HIV, by Luc Montagnier, Françoise Barré-Sinoussi and coworkers
23 May 2011: Telaprevir approved for the treatment of chronic HCV infection
25 May 2011: WHO declared rinderpest eradicated
31 May 1937: First results in humans from the 17D vaccine for yellow fever published by Max Theiler and Hugh H. Smith
Selected intervention
Aciclovir (also acyclovir and sold as Zovirax) is a nucleoside analogue that mimics the nucleoside guanosine. It is active against most viruses in the herpesvirus family, and is mainly used to treat herpes simplex virus infections, chickenpox and shingles. After phosphorylation by viral thymidine kinase and cellular enzymes, the drug inhibits the viral DNA polymerase. Extremely selective and low in cytotoxicity, it was seen as the start of a new era in antiviral therapy. Aciclovir was discovered by Howard Schaeffer and colleagues, and developed by Schaeffer and Gertrude Elion, who was awarded the 1988 Nobel Prize in Medicine in part for its development. Nucleosides isolated from a Caribbean sponge, Cryptotethya crypta, formed the basis for its synthesis. Aciclovir differs from earlier nucleoside analogues in containing only a partial nucleoside structure: the sugar ring is replaced with an open chain. Resistance to the drug is rare in people with a normal immune system.
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