Portal:Viruses
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
Zika fever is caused by the Zika virus, a mosquito-borne flavivirus first isolated in Uganda in 1947. Human infection is usually asymptomatic; a minority of cases have mild symptoms, which generally last 2–7 days and can include fever, conjunctivitis, joint pain, headache and a maculopapular rash. Infections in adults have occasionally been associated with Guillain–Barré syndrome. The incubation period is probably up to a week. The natural reservoir is unknown. The virus is transmitted by Aedes mosquitoes, mainly Aedes aegypti in tropical regions. During pregnancy, mother-to-child transmission can cause microcephaly (pictured) and other brain malformations in some babies. In one study, major abnormalities were seen in 42% of live births. The virus is present in semen and male-to-female sexual transmission has been documented; it is also found in breast milk and blood.
The first documented human outbreak occurred in 2007 in the Federated States of Micronesia. The virus is thought to have entered the Americas in around 2013, and an outbreak started in Brazil in 2015, spreading across the Americas and to Pacific, Asia and Africa, and leading the World Health Organization to consider Zika a Public Health Emergency of International Concern in February–November 2016. No specific treatment nor vaccine has been approved. Prevention involves mosquito control and condom use; women in areas where Zika was circulating were recommended to consider delaying pregnancy, and pregnant women were advised to avoid travel to affected areas.
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
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
Virus classification is the process of naming viruses and placing them into a taxonomic system. They are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms and the type of disease they cause.
Two schemes are in common use. The International Committee on Taxonomy of Viruses (ICTV), established in the early 1970s, classifies viruses into taxa (groups) similar to the biological classification used for cellular organisms, which reflect viruses believed to have a common ancestor. As of 2019, 9 kingdoms, 16 phyla, 36 classes, 55 orders, 168 families, 1,421 genera and 6,589 species of viruses have been defined. Since 2018, viruses have also been classified into higher-level taxa called realms. Four realms are defined, as of 2020, encompassing almost all RNA viruses; some DNA viruses have yet to be assigned a realm.
The older Baltimore classification (pictured), proposed in 1971 by David Baltimore, places viruses into seven groups (I–VII) based on their nucleic acid type, number of strands and sense, as well as the method the virus uses to generate mRNA. There is some concordance between Baltimore groups and the higher levels of the ICTV scheme.
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.
Selected quotation
| “ | To help conceptualize the sheer number of viruses in existence, their current biomass has been estimated to equal that of 75 million blue whales (approximately 200 million tonnes) and, if placed end to end, the collective length of their virions would span 65 galaxies. | ” |
—Peter Simmonds
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
Adenoviruses are a group of non-enveloped DNA viruses that make up the Adenoviridae family. At 90–100 nm in diameter, they are the largest known viruses to lack an envelope. Unique spikes or fibres protrude from the icosahedral capsid, with knobs that bind to the receptor on the host cell. The linear double-stranded genome is 26 to 48 kb long, encodes 23 to 46 proteins, and has a 55 kDa protein attached to each end.
Adenoviruses infect a broad range of vertebrates, including humans, livestock, horses, dogs, bats and other mammals, as well as birds and reptiles, and usually cause infections of the upper respiratory tract. The virus is very stable. Transmission can occur by respiratory droplets or via faeces, with swimming pools being a common source of infection. A total of 57 serotypes, grouped into 7 species, have been found in humans; they cause a wide range of illnesses including mild respiratory infections, conjunctivitis and gastroenteritis. Some types have been associated with obesity. In people with an immunodeficiency they can cause life-threatening multi-organ disease. No antiviral treatment or vaccine is generally available; hand washing is the best way to prevent infection. Adenoviruses are an important viral vector for gene therapy. An oncolytic adenovirus has been approved in China for the treatment of head and neck cancer.
Did you know?
- ...that the stripes on tulips (examples pictured) that caused tulip mania were probably caused by a virus, but this was unknown to science at the time?
- ...that epidemiologist Li Lanjuan was the first to propose a lockdown of Wuhan during the 2019–20 coronavirus outbreak?
- ...that the human bocavirus is the fourth most commonly found virus in samples collected from the respiratory system?
- ...that research by Harold Ginsberg on adenoviruses led to the development of gene therapy, in which modified versions of viruses can be used to implant healthy versions of genes to treat disease?
- ...that the Tiverton fire of 1731 resulted in an increased incidence of smallpox?
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
1 April 1911: Peyton Rous showed that a cell-free isolate could transmit sarcoma in chickens, an early demonstration of cancer caused by a virus
7 April 1931: First electron micrograph taken by Ernst Ruska and Max Knoll
8 April 1976: Bacteriophage MS2 (pictured) sequenced by Walter Fiers and coworkers, first viral genome to be completely sequenced
8 April 1990: Death from AIDS of Ryan White, haemophiliac teenager for whom the Ryan White Care Act is named
8 April 1992: Tennis player Arthur Ashe announced that he had been infected with HIV from blood transfusions
9 April 1982: Stanley Prusiner proposed proteinaceous prions as the cause of scrapie
12 April 1955: Success of trial of Jonas Salk's polio vaccine announced
12 April 2013: New order of double-stranded DNA bacteriophages, Ligamenvirales, announced
15 April 1957: André Lwoff proposes a concise definition of a virus
21 April 1989: Discovery of hepatitis C virus by Qui-Lim Choo and colleagues
28 April 1932: First yellow fever vaccine announced at an American Societies for Experimental Biology meeting by Wilbur Sawyer
29 April 2015: PAHO and WHO declared the Americas region free from rubella transmission
30 April 1937: Discovery of Theiler's murine encephalomyelitis virus, later a model for multiple sclerosis research
Selected intervention
Several human papillomavirus (HPV) vaccines, including Cervarix and Gardasil, have been approved to protect against infections with particular types of HPV, associated with cervical and other cancers. All vaccines protect against the high-risk HPV types 16 and 18. Gardasil is a quadrivalent vaccine that additionally 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. It is estimated that the vaccines may prevent 70% of cervical cancer, 80% of anal cancer, 60% of vaginal cancer, 40% of vulvar cancer and possibly some oropharyngeal cancers. Protection lasts for at least 8–9 years. 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 licensed vaccines are subunit vaccines, containing only the L1 capsid protein of the virus, which self-assembles into virus-like particles. They are not effective in people already infected with HPV. Research is ongoing into therapeutic HPV vaccines including the viral oncoproteins, E6 and E7, but none has yet been licensed.
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