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 5,000 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".
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.
Interferons are a group of signalling proteins released by host cells in response to viruses and other pathogens, as well as tumour cells. They belong to a large class of proteins known as cytokines: molecules used for communication between cells to trigger the protective defences of the immune system that help to eradicate pathogens. More than twenty distinct interferon genes and proteins have been identified in animals, including humans.
When a cell is infected by a virus, several virus products, including glycoproteins and viral RNA, stimulate the cell to produce and release interferons. This causes nearby cells to heighten their defences against viral infection, and so interferes with viral replication. Some viruses, including Japanese encephalitis virus, dengue type 2 virus, human cytomegalovirus and Kaposi's sarcoma-associated herpesvirus, have evolved ways to resist interferon's antiviral activity. Interferons also have many other functions in regulating the immune system. They are responsible for some symptoms of infection, such as fever, muscle pain and "flu-like symptoms".
In the news
14 May: In the ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo – now the second-largest Ebola outbreak in history – new cases continue to increase, with 350 reported in the past 3 weeks, and a total of 1739 cases, including 1147 deaths, since the outbreak began in August 2018. WHO
3 May: In the ongoing Rift Valley fever outbreak in the Mayotte Islands in the Comoro group there have been 129 confirmed cases since the outbreak started in November 2018. WHO
2 May: A European observational study in 972 gay male couples finds no HIV transmission with unprotected sex where the HIV-positive partner's virus is fully suppressed by antiretroviral therapy. Lancet
25 April: A major outbreak of African swine fever ongoing in pigs in China since August 2018 has caused the loss of at least 40 million pigs, and the virus (pictured) has also been reported elsewhere in Southeast Asia. BBC
15 April: The directors of WHO and UNICEF warn that the more than 110,000 measles cases reported globally in January–March represent a nearly threefold increase over the same period in 2018. CNN
14 April: In the ongoing chikungunya virus outbreak in Congo, 6,149 suspected cases have been reported since the outbreak began in January, with nearly half in Kouilou Department. WHO
29 March: The filamentous bacteriophage Pf is shown to increase the pathogenicity of its bacterial host, Pseudomonas aeruginosa, an important human pathogen, by protecting it from the immune system in a mouse model. Science
12 March: The plant nanovirus, faba bean necrotic stunt virus – which has a segmented (multi-part) genome, with each of the eight segments being packaged separately – is shown to be able to replicate successfully even when its DNA segments do not all enter the same cell. eLife
8 March: The International Committee on Taxonomy of Viruses ratifies an update to virus classification, creating the Riboviria taxon for all RNA viruses at the new rank of realm. ICTV
5 March: Another case of apparent clearance of HIV from an infected patient after stem-cell therapy is reported. Nature
5 March: A Danish cohort study in more than 650,000 children confirms that vaccination with the measles, mumps and rubella (MMR) vaccine is not associated with autism. Ann Intern Med
20 February: Influenza A viruses that infect bats are shown to use a novel entry route, via the MHC class II HLA-DR isotype, rather than sialic acid. Nature
The 2009 flu pandemic was an influenza pandemic first recognised in Mexico City in March 2009 and declared over in August 2010. It involved a novel strain of H1N1 influenza virus with genes from five different viruses, which resulted when a previous triple reassortment of avian, swine and human influenza viruses further combined with Eurasian swine influenza viruses, leading to the term "swine flu" being used for the pandemic. It was the second pandemic to involve an H1N1 strain, the first being the 1918 "Spanish flu" pandemic.
The global infection rate was 11–21%. This pandemic strain was less lethal than previous ones, killing about 0.01–0.03% of those infected, compared with 2–3% for Spanish flu. Estimates of global fatalities range from 284,500 to 579,000, mainly in Africa and Southeast Asia – not much above the normal seasonal influenza fatalities of 250,000–500,000 – leading to claims that the World Health Organization had exaggerated the danger.
||...the variety of genes on the planet in viruses exceeds, or is likely to exceed, that in all of the rest of life combined.
Sputnik virophage is a DNA virus, discovered in 2008, that infects Acanthamoeba protozoa. It is a satellite virus of the giant mamavirus. It requires mamavirus to infect the cell simultaneously to replicate, hijacking the virus factories that mamavirus 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 other satellite viruses that infect plants, arthropods and mammals is disputed. Other virophages have since been discovered, including the Mavirus, Zamilon and Organic Lake virophages; all infect protists and all rely on nucleocytoplasmic large DNA viruses as helpers. They have been proposed to belong to a new family, Lavidaviridae.
Sputnik's non-enveloped icosahedral capsid is 50 nm in diameter, and contains a circular double-stranded DNA genome of 18.3 kb. Three of its 21 predicted proteins are thought to derive from mamavirus or the related 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?
Jonas Edward Salk (28 October 1914 – 23 June 1995) was an American medical researcher and virologist, best known for developing the first successful polio vaccine.
Unlike most other researchers, Salk focused on creating an inactivated or "killed" virus vaccine, for safety reasons. The vaccine he developed combines three strains of wild-type poliovirus, inactivated with formalin. The field trial that tested its safety and efficacy in 1954 was one of the largest carried out to date, with vaccine being administered to over 440,000 children. When the trial's success was announced, Salk was hailed as a miracle worker and national hero. A little over two years later, 100 million doses of the vaccine had been distributed throughout the US, with few reported adverse effects. An inactivated vaccine based on the Salk vaccine is the mainstay of polio control in many developed countries.
Salk also researched vaccines against influenza and HIV. In 1960, he founded the Salk Institute for Biological Studies research centre in La Jolla, California.
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