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
Shingles, or herpes zoster, is a painful skin rash with blisters that, characteristically, occurs in a stripe limited to just one side of the body. The rash usually heals within 2–5 weeks, but around one in five people experience residual nerve pain for months or years.
Shingles is caused by varicella zoster virus (VZV), an alpha-herpesvirus. Initial VZV infection usually occurs in childhood causing chickenpox. After this resolves, the virus is not eliminated from the body, but remains latent in the nerve cell bodies of the dorsal root or trigeminal ganglia, without causing symptoms. Years or decades later, shingles occurs when virions in a single ganglion reactivate, travel down nerve fibres and infect the skin around the nerve. The shingles rash is restricted to the area of skin supplied by a single spinal nerve, termed the dermatome. Exactly how VZV remains latent in the body, and subsequently reactivates, is unclear.
Around a third of the population will develop shingles. Repeated episodes are rare. In the United States, about half the cases occur in people aged 50 years or older. Vaccination at least halves the risk, and prompt treatment with aciclovir or related antiviral drugs can reduce the severity and duration of the rash.
Selected image
Biosafety level 3 equipment is used for research with viruses such as influenza that can cause serious disease but for which treatment is available. The biosafety cabinet uses HEPA filters to filter viruses out of the air. This researcher is examining reconstructed 1918 pandemic influenza virus, or "Spanish flu".
Credit: CDC (2005)
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
Virus classification is the process of naming viruses and placing them into a taxonomic system. They are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms and the type of disease they cause.
Two schemes are in common use. The International Committee on Taxonomy of Viruses (ICTV), established in the early 1970s, classifies viruses into taxa (groups) similar to the biological classification used for cellular organisms, which reflect viruses believed to have a common ancestor. As of 2019, 9 kingdoms, 16 phyla, 36 classes, 55 orders, 168 families, 1,421 genera and 6,589 species of viruses have been defined. Since 2018, viruses have also been classified into higher-level taxa called realms. Four realms are defined, as of 2020, encompassing almost all RNA viruses; some DNA viruses have yet to be assigned a realm.
The older Baltimore classification (pictured), proposed in 1971 by David Baltimore, places viruses into seven groups (I–VII) based on their nucleic acid type, number of strands and sense, as well as the method the virus uses to generate mRNA. There is some concordance between Baltimore groups and the higher levels of the ICTV scheme.
Selected outbreak
The 1918–20 influenza pandemic, the first of the two involving H1N1 influenza virus, was unusually deadly. It infected 500 million people across the entire globe, with a death toll of 50–100 million (3–5% of the world's population), making it one of the deadliest natural disasters of human history. It has also been implicated in the outbreak of encephalitis lethargica in the 1920s. Despite the nickname "Spanish flu", the pandemic's geographic origin is unknown.
Most influenza outbreaks disproportionately kill young, elderly or already weakened patients; in contrast this predominantly killed healthy young adults. Contemporary medical reports suggest that malnourishment, overcrowded medical facilities and poor hygiene promoted fatal bacterial pneumonia. Some research suggests that the virus might have killed through a cytokine storm, an overreaction of the body's immune system. This would mean the strong immune reactions of young adults resulted in a more severe disease than the weaker immune systems of children and older adults.
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| “ | There is no question that virology has long outgrown the confines of "Microbiology": it is an important discipline in its own right, a fact that is attested by the numerous high quality journals that are exclusively devoted to it. Further, it is primarily virology that has made possible the rapid rise of molecular cell biology. | ” |
—Bill Joklik on founding a society for virology
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
Papillomaviruses are small non-enveloped DNA viruses that make up the Papillomaviridae family. Their circular double-stranded genome is around 8000 nucleotides long. The icosahedral capsid is 55–60 nm in diameter. They infect humans, other mammals and some other vertebrates including birds, snakes, turtles and fish. Around a hundred species are classified into 53 genera. All papillomaviruses replicate exclusively in epithelial cells of stratified squamous epithelium, which forms the skin and some mucosal surfaces, including the lining of the mouth, airways, genitals and anus.
Infection by most papillomaviruses is either asymptomatic or causes small benign tumours known as warts or papillomas. Francis Peyton Rous showed in 1935 that the Shope papilloma virus could cause skin cancer in rabbits – the first time that a virus was shown to cause cancer in mammals – and papillomas caused by some virus types, including human papillomavirus 16 and 18, carry a risk of becoming cancerous if the infection persists. Papillomaviruses are associated with cancers of the cervix, vulva, vagina, penis, oropharynx and anus in humans.
Did you know?
- ...that the protein ASF/SF2 (molecular image pictured) is involved in the development of the human heart and the replication of HIV-1?
- ...that in February 2020, Neil Ferguson and his team believed that significantly more people in China had been infected with the 2019 novel coronavirus than had been reported?
- ...that the outbreak of Ebola in Bundibugyo District in Uganda in 2007 was caused by a novel strain of the Ebola virus?
- ...that virologist John R. Paul blamed better hygiene for polio's spread in the 20th century, saying early exposure to poliovirus would have given immunity?
- ...that a novel human polyomavirus is associated with Merkel cell carcinoma, a rare and highly aggressive form of skin cancer?
Selected biography
Ali Maow Maalin (1954 – 22 July 2013) was a hospital cook and health worker from Merca, Somalia, who is the last person in the world known to be infected with naturally occurring smallpox. Although he worked in the local smallpox eradication programme, he had not been successfully vaccinated. In October 1977, he was infected with the Variola minor strain of the virus while driving two children with smallpox symptoms to quarantine. He did not experience complications and made a full recovery. An aggressive containment campaign was successful in preventing an outbreak, and smallpox was declared to have been eradicated globally by the World Health Organization (WHO) two years later.
In later life, Maalin volunteered for the successful poliomyelitis eradication campaign in Somalia. He worked for WHO as a local coordinator with responsibility for social mobilisation, and spent several years travelling across Somalia, vaccinating children and educating communities. He encouraged people to be vaccinated by sharing his experiences with smallpox. He died of malaria while carrying out polio vaccinations after the reintroduction of poliovirus to the country in 2013.
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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