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
Smallpox is an eradicated infectious disease of humans caused by the Variola major and V. minor viruses. V. major causes a serious disease with a mortality rate of around 30%; V. minor is associated with much milder symptoms and mortality below 1%. The virus is mainly transmitted by the respiratory route but can also be carried on contaminated objects. Smallpox preferentially infects skin cells, resulting in a usually maculopapular rash, and later, raised fluid-filled blisters. Most V. major survivors have permanent scarring, commonly on the face, which can be extensive. Less common long-term complications include blindness resulting from corneal ulceration and scarring, and in young children, limb deformities due to arthritis and osteomyelitis.
Smallpox probably emerged in human populations in about 10,000 BC; the mummified body of Egyptian pharaoh Ramses V shows evidence of smallpox rash. The disease was responsible for an estimated 300–500 million deaths during the 20th century. Smallpox vaccine, the earliest vaccine, was developed in the 18th century. After intensive vaccination campaigns, the last natural infection occurred in 1977. Smallpox was certified the first infectious disease to be eradicated globally in 1979. Debate is ongoing over whether all stocks of the virus should be destroyed.
Selected image
The MS2 bacteriophage was the first virus genome to be sequenced in 1976. Its capsid has an icosahedral structure made up from 180 copies of the coat protein.
Credit: Neil Ranson (7 June 2011)
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
Reverse transcriptase is an enzyme that generates complementary DNA from an RNA template, in contrast to the usual information flow from DNA to RNA. It was discovered in Rous sarcoma virus and murine leukaemia virus, two retroviruses, by Howard Temin and David Baltimore, working independently in 1970, for which the two shared the Nobel Prize in Physiology or Medicine.
Reverse transcription is essential for the replication of retroviruses, allowing them to integrate into the host genome as a provirus. The enzyme is a target for reverse-transcriptase inhibitors, a major class of anti-HIV drugs. Reverse transcription is also used by Hepadnaviridae and Caulimoviridae, DNA viruses that replicate via an RNA intermediate, such as hepatitis B. The process is important in the movement of retrotransposons, a type of mobile genetic element, and in the extension of chromosome ends in eukaryotic genomes. The enzyme is widely used in the laboratory for molecular cloning, RNA sequencing, polymerase chain reaction and genome analysis.
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.
Selected quotation
| “ | ...the variety of genes on the planet in viruses exceeds, or is likely to exceed, that in all of the rest of life combined. | ” |
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
Henipaviruses are a genus of RNA viruses in the Paramyxoviridae family. The variably shaped, 40–600 nm diameter, enveloped capsid contains a single-stranded, negative-sense RNA genome of 18.2 kb, with six genes. The cellular receptor is in the ephrin family. The natural hosts are predominantly bats, mainly the Pteropus genus of megabats (flying foxes) and some microbats. Bats infected with Hendra virus develop viraemia and shed virus in urine, faeces and saliva for around a week, but show no signs of disease. Henipaviruses can also infect humans and livestock, causing severe disease with high mortality, making the group a zoonootic disease. Transmission to humans sometimes occurs via an intermediate domestic animal host.
The first henipavirus, Hendra virus, was discovered in 1994 as the cause of an outbreak in horses in Brisbane, Australia. Nipah virus was identified a few years later in Malaysia as the cause of an outbreak in pigs. Three further species have since been recognised: Cedar and Kumasi viruses in bats, and Mòjiāng virus in rodents. Their emergence as human pathogens has been linked to increased contact between bats and humans. Human disease has been confined to Australia and Asia, but members of the genus have also been found in African bats. A veterinary vaccine against Hendra virus is available but no human vaccine has been licensed.
Did you know?
- ...that the largest in-flight transmission of SARS virus (coronavirus pictured) took place on an Air China flight in March 2003?
- ...that computational biologist Bette Korber describes her development of a mosaic antigen vaccine against HIV as creating "little Frankenstein proteins"?
- ...that although measles was declared eradicated from the United States in 2000, more than 500 people there had been diagnosed with the disease in January–May 2019?
- ...that Barrie Marmion spearheaded the development of the first vaccine against Q fever?
- ...that Michael Osterholm and Mark Olshaker's book Deadliest Enemy provides a nine-point plan to defeat "killer germs"?
Selected biography
Thomas Flewett (29 June 1922 – 12 December 2006) was a British–Irish virologist and an authority on electron microscopy of viruses, best known for his role in the discovery of rotaviruses. After Ruth Bishop and others discovered viruses associated with diarrhoea, Flewett showed that they could be visualised by electron microscopy directly in faeces. He dubbed them "rotaviruses" for their wheel-shaped appearance. His group described the different rotavirus serotypes, and did extensive research on the rotavirus varieties infecting many animals.
Flewett established one of the first English virus laboratories in Birmingham in 1956. In addition to his rotavirus work, he discovered the cause of hand, foot and mouth disease, identified two new species of adenovirus, and co-discovered human torovirus and picobirnaviruses. His other research included influenza, coxsackie A, coxsackie B and hepatitis B viruses.
In this month
1 December 1988: First World AIDS Day
4 December 1915: Frederick Twort discovered bacteriophages
4 December 2009: New order of single-stranded RNA viruses, Tymovirales, announced
6 December 1995: Saquinavir approved by FDA; the first HIV protease inhibitor
6 December 2013: Sofosbuvir approved for treatment of chronic hepatitis C virus (HCV), the first HCV nucleotide analogue and the first drug approved for interferon-free treatment
9 December 1979: Global Commission for Certification of Smallpox Eradication signed document formally certifying smallpox eradication
15 December 1955: Crystallisation of poliovirus by Fred Schaffer and Carlton Schwerdt, the first animal virus to be crystallised
15 December 1967: Infectious phi X 174 synthesised by Arthur Kornberg and coworkers, the first synthetic virus
18 December 1908: Poliovirus discovered by Karl Landsteiner and Erwin Popper
25 December 1982: Lambda phage (plaques pictured) sequenced by Fred Sanger and coworkers
28 December 1936: Scrapie shown to be transmissible, the first demonstration for a prion disease
29 December 1926: Thomas Milton Rivers proposed that viruses are obligate parasites
Selected intervention
Oseltamivir (also Tamiflu) is an oral antiviral drug against influenza (flu). It was the second inhibitor of the viral neuraminidase to be developed, after zanamivir, and the first to be taken as an oral tablet. It was originally synthesised from shikimic acid extracted from the star anise plant. Oseltamivir is a prodrug that requires metabolism in the liver to the active form, oseltamivir carboxylate. This binds at the active site of the neuraminidase enzyme, preventing it from cleaving sialic acid to release the virus particle from the host cell. Oseltamivir can reduce the duration of influenza symptoms by 0.5–1 days. Debate is ongoing about whether it also reduces the risk of complications, such as pneumonia. Nausea and vomiting are the main adverse events. Resistance to oseltamivir has been observed in some strains of influenza virus, especially H1N1 strains.
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