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
Meningitis is an acute inflammation of the meninges, protective membranes covering the brain and spinal cord. Symptoms in adults include headache, fever and neck stiffness, as well as sometimes confusion or altered consciousness, vomiting, and an inability to tolerate light or loud noises. Children often show only nonspecific symptoms, such as irritability or drowsiness.
The most common cause is infection with viruses including enteroviruses, herpes simplex virus (mainly HSV-2), varicella zoster virus, mumps virus, HIV and lymphocytic choriomeningitis. In Western countries, viral meningitis occurs in around 11 people per 100,000 each year. Infection with bacteria, fungi, protozoa and parasites can also cause meningitis, and there are several non-infectious causes. Although some forms of meningitis can be life-threatening, viral meningitis is generally more benign than that caused by bacterial infection. It usually resolves spontaneously and is rarely fatal. HSV-2 can cause a chronic, recurrent form called Mollaret's meningitis.
Polymerase chain reaction of cerebrospinal fluid and identification of antibodies can be used to differentiate between viral causes. Viral meningitis typically only requires supportive therapy; meningitis caused by HSV or varicella zoster virus sometimes responds to treatment with antiviral drugs such as aciclovir. Mumps-associated meningitis can be prevented by vaccination.
Selected image
Eastern equine encephalitis virus is an Alphavirus that is transmitted between birds and mammals, including humans and horses, by several mosquito species. The virus (coloured in red) is found in the mosquito salivary gland, and is injected into the new host when the insect feeds.
Credit: Fred Murphy, Sylvia Whitfield, CDC (1968)
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
Plant viruses face different challenges from animal viruses. As plants do not move, transmission between hosts often involves vectors, most commonly insects, but also nematodes and protozoa. Plant viruses can also spread via seeds, and by direct transfer of sap. Plant cells are surrounded by cell walls which are difficult to penetrate. Movement between cells occurs mainly by transport through plasmodesmata, and most plant viruses encode movement proteins to make this possible. Although plants lack an adaptive immune system, they have complex defences against viral infection. Viruses of cultivated plants often cause disease, and are thought to cause up to US$60 billion losses to global crop yields each year; infections of wild plants are often asymptomatic.
Most plant viruses are rod-shaped, with protein discs forming a tube surrounding the viral genome; isometric particles are another common structure. They rarely have an envelope. The great majority have an RNA genome, which is usually small and single stranded. Plant viruses are grouped into 73 genera and 49 families. Tobacco mosaic virus (pictured) is among the best characterised of the 977 species officially recognised in 1999.
Selected outbreak
The last recorded smallpox death occurred during the 1978 smallpox outbreak in Birmingham, UK. The outbreak resulted from accidental exposure to the Abid strain of Variola major, from a laboratory, headed by Henry Bedson, at the University of Birmingham Medical School – also associated with an outbreak in 1966. Bedson was investigating strains of smallpox known as whitepox, considered a potential threat to the smallpox eradication campaign, then in its final stages.
A medical photographer who worked on the floor above the laboratory showed smallpox symptoms in August and died the following month; one of her contacts was also infected but survived. The government inquiry into the outbreak concluded that she had been infected in late July, possibly via ducting, although the precise route of transmission was subsequently challenged. The inquiry criticised the university's safety procedures. Bedson committed suicide while under quarantine. Radical changes in UK research practices for handling dangerous pathogens followed, and all known stocks of smallpox virus were concentrated in two laboratories.
Selected quotation
| “ | A virus is not an individual organism in the ordinary sense of the term, but something which could almost be called a stream of biological patterns. | ” |
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
Poliovirus is an enterovirus, an RNA virus in the Picornaviridae family, associated with the paralytic disease polio. The icosahedral virus particle is about 30 nanometres in diameter and lacks an envelope. It contains a relatively short, single-stranded positive RNA genome of around 7500 nucleotides, which encodes about ten viral products. The virus has a fairly high mutation rate even for an RNA virus. Historically there were three serotypes, each with a slightly different capsid protein; PV1 is the most common, and PV2 was declared eradicated in 2015.
The virus only naturally infects humans, although some monkeys can be infected experimentally; 95% of infections are asymptomatic. Infection occurs via the faecal–oral route and viral replication occurs in the alimentary tract. The virus enters the host cell by binding to an immunoglobulin-like receptor, CD155. Fully assembled poliovirus leaves the cell 4–6 hours after initiation of infection. Poliovirus was first isolated in 1909 by Karl Landsteiner and Erwin Popper. Its genome was sequenced in 1981. Among the simplest clinically significant viruses, poliovirus is one of the best-characterised viruses, and has become a useful model for studying RNA viruses.
Did you know?
- ...that virologist Harald zur Hausen received both the Gairdner Foundation International Award and the Nobel Prize in Physiology or Medicine in 2008?
- ...that in 2007, the first Australian outbreak of equine influenza caused a nationwide ban on horse racing?
- ...that Michael S. Gottlieb was instrumental in identifying AIDS in 1981 and thereafter treated actor Rock Hudson for the disease?
- ...that the tick Ornithodoros erraticus spreads the African swine fever virus in the Iberian Peninsula (Spain and Portugal)?
- ...that the CEO of CEPI, a key organisation in the race to develop a vaccine for COVID-19, has called the disease "the most frightening" that he has ever encountered in his career?
Selected biography
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.
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
Aciclovir (also acyclovir and sold as Zovirax) is a nucleoside analogue that mimics the nucleoside guanosine. It is active against most viruses in the herpesvirus family, and is mainly used to treat herpes simplex virus infections, chickenpox and shingles. After phosphorylation by viral thymidine kinase and cellular enzymes, the drug inhibits the viral DNA polymerase. Extremely selective and low in cytotoxicity, it was seen as the start of a new era in antiviral therapy. Aciclovir was discovered by Howard Schaeffer and colleagues, and developed by Schaeffer and Gertrude Elion, who was awarded the 1988 Nobel Prize in Medicine in part for its development. Nucleosides isolated from a Caribbean sponge, Cryptotethya crypta, formed the basis for its synthesis. Aciclovir differs from earlier nucleoside analogues in containing only a partial nucleoside structure: the sugar ring is replaced with an open chain. Resistance to the drug is rare in people with a normal immune system.
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