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
Chickenpox is caused by initial infection with varicella zoster virus, a DNA virus of the Alphaherpesvirinae subfamily. The virus naturally infects only humans, but some other primates have been infected artificially. Symptoms appear 10–21 days after exposure: an itchy vesicular skin rash, and small ulcers in the oral cavity and tonsil areas. The rash usually resolves by 7 days, but the virus remains latent in nerve cell bodies, and can emerge years or decades later to cause shingles. Chickenpox is transmitted by the respiratory route, as well as direct contact with lesions.
A classic disease of childhood, the highest prevalence occurs at 4–10 years. Chickenpox is rarely fatal in people with a normal immune system, with around 6,400 deaths worldwide in 2015, about 1 in 60,000 infections. Adults often have more severe symptoms than children, and are at higher risk of complications such as pneumonia, bronchitis, hepatitis and encephalitis. Pregnant women and people with a suppressed immune system have the highest complication risk. Chickenpox during the first 28 weeks of gestation can lead to foetal malformations. Infection in adults is usually treated with antiviral drugs, such as aciclovir or valaciclovir, which reduces symptom severity and the risk of complications. A vaccine is available.
Selected image
Chikungunya virus is an alphavirus transmitted by Aedes mosquitoes. The disease can cause severe joint pain, sometimes lasting for several months. Outbreaks have occurred across Africa, Asia and India, and in 2013–14, in South America and the Caribbean.
Credit: A2-33 (8 December 2013)
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
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The Hershey–Chase experiments were conducted by Alfred Hershey and Martha Chase in 1952 using the T2 bacteriophage (pictured), which is composed of DNA wrapped in a protein shell. Hershey and Chase labelled either the phage DNA using radioactive phosphorus-32 or the protein using radioactive sulphur-35. They allowed the radiolabelled phages to infect unlabelled bacteria, and then agitated in a blender and centrifuged to separate material remaining outside the bacterial cells. The majority of the 32P-labelled DNA entered the host bacterial cell, while all the 35S-labelled protein remained outside. Hershey and Chase also showed that the phage DNA is inserted into the bacteria shortly after the virus attaches to its host.
DNA had been known since 1869, but in 1952 many scientists believed that proteins carried the information for inheritance. Proteins appeared more complex, while DNA was thought to be an inert molecule used for phosphorus storage. These experiments built on earlier research on transformation in bacteria and helped to confirm that DNA, not protein, was the genetic material. Hershey shared the 1969 Nobel Prize in Physiology or Medicine for the research.
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
| “ | We are part virus. This bizarre yet inescapable fact has been revealed over the past 30 years, as scientists have spelunked their way through the human genome and encountered stretches of DNA with the telltale chemical signatures of viruses. | ” |
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
Hantaviruses (or orthohantaviruses) are a family of RNA viruses in the Bunyavirales order. The enveloped virion is 120–160 nm in diameter and contains a single-stranded, negative-sense RNA genome with three segments. They infect many species of rodents, as well as shrews and moles, without causing disease, and can be transmitted to humans, where they can cause serious disease. Hantaan virus, the first known hantavirus, was discovered in 1976 as the cause of a novel haemorrhagic fever affecting combatants in the Korean War; it can also be caused by other hantaviruses, including Dobrava-Belgrade and Seoul viruses. Some hantaviruses, including Sin Nombre and Bayou, cause a pulmonary syndrome. Others have not yet been associated with human disease.
Unlike other bunyaviruses, hantaviruses are not transmitted by arthropods. Rodents act as the vector, with transmission to humans usually occurring via contact with urine, saliva or faeces, by inhalation of aerosolised excreta or by bite. Little is understood about how hantaviruses cause disease; the main site of viral replication in the body is unknown. Rodent control is important in disease prevention.
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
Frederick Sanger (13 August 1918 – 19 November 2013) was a British biochemist, the only person to have been awarded the Nobel Prize in Chemistry twice.
He started to research nucleic acid sequencing in the early 1960s. In 1975, he co-invented the "Plus and Minus" technique for sequencing DNA, which could sequence 80 nucleotides at once, a significant improvement on earlier techniques. Using this method, his group sequenced most of the 5,386 nucleotides of φX174 bacteriophage – the first virus and the first DNA genome to be completely sequenced – and showed that some of its genes overlapped. In 1977, he and his group pioneered the Sanger (or dideoxy chain-termination) method for sequencing DNA and used it to sequence the 48,502 bp bacteriophage λ. His technique remained the most widely used sequencing method until the mid-2000s, and was used to generate the first human genome sequence. Sanger is also known for sequencing bovine insulin, the first protein to be sequenced. The Sanger Institute was named for him.
In this month
March 1990: Proposal for a database of all viruses, later the ICTVdB
3 March 2014: Discovery of Pithovirus sibericum, the largest known virus at 1.5 μm long by 0.5 μm in diameter
4 March 1918: First case reported in the 1918 influenza pandemic
10 March 1956: Francis Crick and James Watson proposed that small viruses have a protein shell consisting of a large number of identical subunits
10 March 1956: Donald Caspar published paper on the structure of tomato bushy stunt virus
13 March 2003: Enfuvirtide (T20) approved; first HIV fusion inhibitor, also first HIV entry inhibitor
20 March 1987: Antiretroviral drug AZT (pictured) became the first antiviral medication approved for use against HIV/AIDS
22 March 2014: First case reported in the West African Ebola outbreak, the most widespread so far
26 March 1953: Jonas Salk reported a successful test of an inactivated polio vaccine.
28 March 2003: Mimivirus shown to be a virus, then the largest known
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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