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".
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Measles is a disease that only affects humans caused by the measles virus, an RNA virus in the Paramyxoviridae family. It is highly contagious, with transmission occurring via the respiratory route or by contact with secretions. Symptoms generally develop 10–12 days after exposure and last 7–10 days; they include high fever, cough, rhinitis and conjunctivitis, white Koplik's spots inside the mouth and a generalised red maculopapular rash. Complications including diarrhoea, otitis media and pneumonia are relatively common; more rarely seizures, encephalitis, croup, corneal ulceration and blindness can occur. The risk of death is usually around 0.2%, but may be as high as 10–28% in areas with high levels of malnutrition and poor healthcare.
Measles was first described by Rhazes (860–932). The disease is estimated to have killed around 200 million people between 1855 and 2005. It affects about 20 million people a year, primarily in the developing areas of Africa and Asia, and is one of the leading vaccine-preventable disease causes of death. No antiviral drug is licensed. An effective measles vaccine is available, but uptake has been reduced by anti-vaccination campaigns, particularly the fraudulent claim that the MMR vaccine might be associated with autism. Rates of disease and deaths increased from 2017 to 2019, attributed to a decrease in immunisation.
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Lady Mary Wortley Montagu survived smallpox, and popularised the Turkish practice of variolation against the disease in western Europe in the 1720s.
Credit: Jean-Étienne Liotard (c. 1756)
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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Plant viruses face different challenges from animal viruses. As plants do not move, transmission between hosts often involves vectors, most commonly insects, but also nematodes and protozoa. Plant viruses can also spread via seeds, and by direct transfer of sap. Plant cells are surrounded by cell walls which are difficult to penetrate. Movement between cells occurs mainly by transport through plasmodesmata, and most plant viruses encode movement proteins to make this possible. Although plants lack an adaptive immune system, they have complex defences against viral infection. Viruses of cultivated plants often cause disease, and are thought to cause up to US$60 billion losses to global crop yields each year; infections of wild plants are often asymptomatic.
Most plant viruses are rod-shaped, with protein discs forming a tube surrounding the viral genome; isometric particles are another common structure. They rarely have an envelope. The great majority have an RNA genome, which is usually small and single stranded. Plant viruses are grouped into 73 genera and 49 families. Tobacco mosaic virus (pictured) is among the best characterised of the 977 species officially recognised in 1999.
Selected outbreak
The West African Ebola epidemic was the most widespread outbreak of the disease to date. Beginning in Meliandou in southern Guinea in December 2013, it spread to adjacent Liberia and Sierra Leone, affecting the cities of Conakry and Monrovia, with minor outbreaks in Mali and Nigeria. Cases reached a peak in October 2014 and the epidemic was under control by late 2015, although occasional cases continued to occur into April 2016. Ring vaccination with the then-experimental vaccine rVSV-ZEBOV was trialled in Guinea.
More than 28,000 suspected cases were reported with more than 11,000 deaths; the case fatality rate was around 40% overall and around 58% in hospitalised patients. Early in the epidemic nearly 10% of the dead were healthcare workers. The outbreak left about 17,000 survivors, many of whom reported long-lasting post-recovery symptoms. Extreme poverty, dysfunctional healthcare systems, distrust of government after years of armed conflict, local burial customs of washing the body, the unprecedented spread of Ebola to densely populated cities, and the delay in response of several months all contributed to the failure to control the epidemic.
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| “ | There are more viruses on Earth than stars in the universe. | ” |
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
Sputnik virophage is a subviral agent, discovered in 2008, that infects Acanthamoeba protozoa. It is a satellite virus of giant viruses of the Mimiviridae family. It requires a mimivirus to infect the cell simultaneously to replicate, hijacking the virus factories that mimivirus creates and impairing its replication. Sputnik was the first satellite to be shown to inhibit the replication of its associated helper virus. Such viruses have been termed "virophages" or "virus eaters" – by analogy with bacteriophages, viruses that parasitise bacteria – but the distinction between virophages and classical satellite viruses that infect plants, arthropods and mammals is disputed. Three Sputnik types are now known, and other virophages have since been discovered, now classified in the Lavidaviridae family, including the Zamilon, Mavirus and Organic Lake virophages. All virophages that have been characterised infect protists and all rely on nucleocytoplasmic large DNA viruses as helpers.
Sputnik's non-enveloped icosahedral capsid is 74 nm in diameter, and contains a circular double-stranded DNA genome of 18.3 kb. Three of its 21 predicted protein-coding genes are thought to derive from Acanthamoeba polyphaga mimivirus, suggesting that virophages and giant viruses can swap genes during their joint infection of Acanthamoeba, and also that virophages might mediate horizontal gene transfer between giant viruses.
Did you know?
- ...that the groundnut rosette virus, which causes serious damage to peanut crops in Africa (plant pictured), is spread by the groundnut aphid?
- ...that Oguntola Sapara infiltrated and exposed a secret society in Nigeria that was deliberately spreading smallpox?
- ...that with the 2008 bird flu outbreak in West Bengal, 16,000 birds were destroyed in Itahar, but health workers retreated from villages that refused to kill their birds?
- ...that Preparing for Emergencies was a British Home Office programme to increase public safety after several major disasters, including the Madrid bombings, SARS outbreak and UK foot and mouth crisis?
- ...that Sounds of HIV sets the genome of the HIV/AIDS virus to music?
Selected biography
Rosalind Elsie Franklin (25 July 1920 – 16 April 1958) was a British biophysicist and X-ray crystallographer who made critical contributions to the understanding of the fine molecular structures of DNA, RNA and viruses.
Franklin led pioneering research on the structure of tobacco mosaic virus (TMV), a rod-like RNA virus, using X-ray crystallography. She first showed that, contrary to contemporary opinion, TMV virus particles were all of the same length. With Kenneth Holmes, she showed the virus's coat is composed of protein molecules arranged in helices. She designed and built a model of the virus to be exhibited at the 1958 World's Fair. She speculated that the virus is hollow, and correctly hypothesized that the RNA of TMV is single-stranded. Her work, together with that of Donald Caspar, revealed that the viral RNA is wound along the inner surface of the hollow virus. Her laboratory, which also included Aaron Klug, studied other plant viruses, including turnip yellow mosaic virus and viruses infecting potato, tomato and pea. Franklin also worked on icosahedral animal viruses, including poliovirus.
Franklin is commemorated in the Rosalind Franklin University of Medicine and Science.
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
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The first Ebola vaccine was approved in 2019. Developed by the Public Health Agency of Canada, rVSV-ZEBOV is based on an attenuated recombinant vesicular stomatitis virus, genetically modified to express a surface glycoprotein of Zaire ebolavirus, and is estimated to be 97.5% effective. In the Kivu Ebola epidemic of 2018–20, a ring vaccination strategy was employed to protect direct and indirect contacts of infected people, as well as health workers, and around 300,000 people were vaccinated with rVSV-ZEBOV. A second vaccine was approved in 2020; this uses two different doses – a vector based on human adenovirus serotype 26 used to prime, boosted around eight weeks later by modified vaccinia Ankara (based on a heavily attenuated vaccinia virus) – and is not suitable for response to an outbreak. The efficacy is unknown. Multiple other vaccine candidates are in development to prevent Ebola, including replication-deficient adenovirus vectors, replication-competent human parainfluenza 3 vectors, and virus-like nanoparticle preparations.
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