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
Variant Creutzfeldt–Jakob disease, or vCJD, is a rare type of central nervous system disease within the transmissible spongiform encephalopathy family, caused by a prion. First identified in 1996, vCJD is now distinguished from classic CJD. The incubation period is believed to be years, possibly over 50 years. Prion protein can be detected in appendix and lymphoid tissue (pictured) up to two years before the onset of neurological symptoms, which include psychiatric problems, behavioural changes and painful sensations. Abnormal prion proteins build up as amyloid deposits in the brain, which acquires a characteristic spongiform appearance, with many round vacuoles in the cerebellum and cerebrum. The average life expectancy after symptoms start is 13 months.
About 170 cases have been recorded in the UK, and 50 cases in the rest of the world. The estimated prevalence in the UK is about 1 in 2000, higher than the reported cases. Transmission is believed to be mainly from consuming beef contaminated with the bovine spongiform encephalopathy prion, but may potentially also occur via blood products or contaminated surgical equipment. Infection is also believed to require a specific genetic susceptibility in the PRNP-encoding gene. Human PRNP protein can have either methionine or valine at position 129; nearly all of those affected had two copies of the methionine-containing form, found in 40% of Caucasians.
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
A global drive to eradicate poliovirus started in 1988, when there were an estimated 350,000 cases of wild poliovirus infection globally. Two diseases, both caused by viruses, have been eradicated, smallpox in 1980 and rinderpest in 2011. Poliovirus only infects humans. It persists in the environment for a few weeks at room temperature and a few months at 0–8 °C. The oral polio vaccine is inexpensive, highly effective and is predicted to generate lifelong immunity. Reversion of live vaccine strains to virulence has resulted in occasional cases of vaccine-associated polio paralysis.
Two of the three strains of wild-type poliovirus have been eradicated. Annual reported cases of wild poliovirus infection fell to a low of 22 in 2017, but had risen to 176 in 2019. As of 2020, the wild virus remains endemic only in Afghanistan and Pakistan, but vaccine-derived poliovirus is circulating in several countries. A lack of basic health infrastructure and civil war remain significant obstacles to eradication. Some local communities have opposed immunisation campaigns, and vaccination workers have been murdered in Pakistan and Nigeria.
Selected outbreak
The 1993 hantavirus outbreak in the Four Corners region of southwest USA was of a novel hantavirus, subsequently named Sin Nombre virus. It caused the previously unrecognised hantavirus pulmonary syndrome – the first time that a hantavirus had been associated with respiratory symptoms. Mild flu-like symptoms were followed by the sudden onset of pulmonary oedema, which was fatal in half of those affected. A total of 24 cases were reported in April–May 1993, with many of those affected being from the Navajo Nation territory. Hantavirus infection of humans generally occurs by inhaling aerosolised urine and faeces of rodents, in this case the deer mouse (Peromyscus; pictured).
Previously documented hantavirus disease had been confined to Asia and Europe, and these were the first human cases to be recognised in the USA. Subsequent investigation revealed undiagnosed cases dating back to 1959, and Navajo people recalled similar outbreaks in 1918, 1933 and 1934.
Selected quotation
| “ | The unusual features of the giant Mimivirus revived the popular, yet unresolved question: "Are viruses alive?" The discovery that some of them can get sick adds a new twist to this old debate. | ” |
—Hiroyuki Ogata & Jean-Michel Claverie on the relationship between Sputnik virophage and mimivirus
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
Tobacco mosaic virus (TMV) is an RNA virus in the Virgaviridae family that infects a wide range of plants, including tobacco, tomato, pepper, other members of the Solanaceae family, and cucumber. The rod-shaped virus particle is around 300 nm long and 18 nm in diameter, and consists of a helical capsid made from 2130 copies of a single coat protein, which is wrapped around a positive-sense single-stranded RNA genome of around 6400 bases. The coat protein and RNA can self-assemble to produce infectious virus.
Infection often causes characteristic patterns, such as "mosaic"-like mottling and discoloration on the leaves, but is almost symptomless in some host species. TMV causes an economically important disease in tobacco plants. Transmission is frequently by human handling, and prevention of infection involves destroying infected plants, hand washing and crop rotation to avoid contaminated soil. TMV is one of the most stable viruses known. The fact that it does not infect animals and can readily be produced in gramme amounts has led to its use in numerous pioneering studies in virology and structural biology. TMV was the first virus to be discovered and the first to be crystallised.
Did you know?
- ...that the rice rat Oryzomys couesi (pictured; top) is infected by two different hantaviruses?
- ...that Chinese doctor Kang Laiyi spent more than 30 years researching the epidemiology of HIV?
- ...that in the 2014 edition of Indonesia's Jakarta Marathon, a limited number of African athletes were invited because of the 2014 Ebola outbreak in Africa?
- ...that the veterinarian Janina Oyrzanowska-Poplewska, whose work led to the creation of the first Polish canine distemper vaccine, has been honoured as a Righteous Among the Nations?
- ...that experts on the World Health Organization's emergency committee believe that COVID-19 could be the first Disease X?
Selected biography
Françoise Barré-Sinoussi (born 30 July 1947) is a French virologist, known for being one of the researchers who discovered HIV.
Barré-Sinoussi researched retroviruses in Luc Montagnier's group at the Institut Pasteur in Paris. In 1982, she and her co-workers started to analyse samples from people with a new disease, then referred to as "gay-related immune deficiency". They found a novel retrovirus in lymph node tissue, which they called "lymphadenopathy-associated virus". Their results were published simultaneously with those of Robert Gallo's group in the USA, who had independently discovered the virus under the name "human T-lymphotropic virus type III". The virus, renamed "human immunodeficiency virus", was later shown to cause AIDS. Barré-Sinoussi continued to research HIV until her retirement in 2015, studying how the virus is transmitted from mother to child, the immune response to HIV, and how a small proportion of infected individuals, termed "long-term nonprogressors", can limit HIV replication without treatment. In 2008, she was awarded the Nobel Prize, with Montagnier, for the discovery of HIV.
In this month
5 June 1981: First report of HIV/AIDS (symbol pictured) appeared in medical literature
6 June 1997: Gene silencing in plants shown to be a viral defence mechanism
7–13 June 1962: Donald Caspar and Aaron Klug proposed the quasi-equivalence principle of virus structure
7–13 June 1962: André Lwoff proposed a viral classification scheme based on nature of genome, type of symmetry and presence of envelope
7–13 June 1962: George Hirst proposed that the influenza virus genome is segmented
9 June 1981: The American Society for Virology was founded
13 June 2012: First case of Middle East respiratory syndrome coronavirus (MERS-CoV) occurred in Saudi Arabia
18 June 1981: A vaccine against foot-and-mouth disease was the first genetically engineered vaccine
21 June 1996: Nevirapine approved, first NNRTI for HIV/AIDS
26 June 1993: Clinical trial of hepatitis B virus drug fialuridine terminated; the drug caused several fatalities due to lactic acidosis
28 June 2011: FAO declared rinderpest eradicated
30 June 1985: Ryan White was denied re-admittance to his school after an AIDS diagnosis, in a case that changed public perceptions of the disease
Selected intervention
Ribavirin is a nucleoside analogue that mimics the nucleoside guanosine. It shows some activity against a broad range of DNA and RNA viruses, but is less effective against dengue fever, yellow fever and other flaviviruses. The drug was first synthesised in the early 1970s by Joseph T. Witkowski and Roland K. Robins. Ribavirin's main current use is against hepatitis C, in combination with pegylated interferon, nucleotide analogues and protease inhibitors. It has been used in the past in an aerosol formulation against respiratory syncytial virus-related diseases in children. Ribavirin has been used in combination as part of an experimental treatment for rabies. It is also the only available treatment for the viruses causing some viral haemorrhagic fevers, including Lassa fever, Crimean–Congo haemorrhagic fever and hantavirus disease, but is ineffective against the filovirus diseases, Ebola and Marburg. Clinical use is limited by the drug building up in red blood cells to cause haemolytic anaemia.
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