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
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Herpes simplex is caused by herpes simplex virus types 1 and 2 of the Herpesviridae family, with 60–95% of adults being infected with one of the types. Common forms of infection are oral herpes, which can result in cold sores, and genital herpes. Active disease often involves blisters containing infectious virus, although the genital form is frequently asymptomatic. Less common disorders associated with the viruses include herpetic whitlow, herpes gladiatorum, ocular herpes, herpesviral encephalitis and Mollaret's meningitis.
After initial infection, virus particles are transported along sensory nerves to the cell bodies in the ganglion, where they become latent and remain lifelong. Periods of remission alternate with outbreaks of active disease, in which the virus multiplies in the nerve cell and new virus particles are transported along the nerve fibre to the nerve terminals in the skin, where they are released. What causes these recurrences is unclear. Transmission is usually by direct contact with a lesion or with body fluids, and can occur during periods of asymptomatic shedding. Neonatal herpes is possible after transmission from the mother. Barrier protection methods reduce genital herpes risk. No vaccine or cure exists, but antiviral treatment can alleviate symptoms and reduce viral shedding.
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The common cold is the most frequent infectious disease. Despite the advice to "consult your physician" no antiviral treatment has been approved, and colds are only rarely associated with serious complications.
Credit: Federal Art Project (1937)
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
RNA interference is a type of gene silencing that forms an important part of the immune response against viruses and other foreign genetic material in plants and many other eukaryotes. A cell enzyme called Dicer (pictured) cleaves double-stranded RNA molecules found in the cell cytoplasm – such as the genome of an RNA virus or its replication intermediates – into short fragments termed small interfering RNAs (siRNAs). These are separated into single strands and integrated into a large multi-protein RNA-induced silencing complex, where they recognise their complementary messenger RNA (mRNA) molecules and target them for destruction. This prevents the mRNAs acting as a template for translation into proteins, and so inhibits, or silences, the expression of viral genes.
RNA interference allows the entire plant to respond to a virus after a localised encounter, as the siRNAs can transfer between cells via plasmodesmata. The protective effect can be transferred between plants by grafting. Many plant viruses have evolved elaborate mechanisms to suppress this response. RNA interference evolved early in eukaryotes, and the system is widespread. It is important in innate immunity towards viruses in some insects, but relatively little is known about its role in mammals. Research is ongoing into the application of RNA interference to antiviral treatments.
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The 1976 Zaire Ebola virus outbreak was one of the first two recorded outbreaks of the disease. The causative agent was identified as a novel virus, named for the region's Ebola River. The first identified case, in August, worked in the school in Yambuku, a small rural village in Mongala District, north Zaire. He had been treated for suspected malaria at the Yambuku Mission Hospital, which is now thought to have spread the virus by giving vitamin injections with inadequately sterilised needles, particularly to women attending prenatal clinics. Unsafe burial practices also spread the virus.
The outbreak was contained by quarantining local villages, sterilising medical equipment and providing protective clothing to medical personnel, and was over by early November. A total of 318 cases was recorded, of whom 280 died, an 88% case fatality rate. An earlier outbreak in June–November in Nzara, Sudan, was initially thought to be linked, but was shown to have been caused by a different species of Ebola virus.
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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
Adeno-associated viruses (AAVs) are two small DNA viruses in the Dependoparvovirus genus of the Parvoviridae family. They cannot complete their lytic replication cycle without a helper virus, which include adenoviruses, herpesviruses and vaccinia. In the absence of the helper, AAVs can integrate into the host genome at a specific site on human chromosome 19, or persist as an episome. The 20 nm icosahedral capsid lacks an envelope, and contains a single-stranded DNA genome of around 4.7 kb. AAVs infect humans and some other primates without causing disease. They generate only a mild immune response, including neutralising antibodies. The best-studied of the 11 serotypes, AAV-2, infects nerve cells, liver cells, skeletal muscle and vascular smooth muscle, using heparan sulphate proteoglycan as its primary receptor.
Its low pathogenicity makes AAV an attractive basis for viral vectors for gene therapy. Alipogene tiparvovec to treat lipoprotein lipase deficiency was the first gene therapy to be licensed, but was later withdrawn. Promising results have been obtained in early clinical trials with AAV-based gene therapy in haemophilia, congestive heart failure, spinal muscular atrophy, Parkinson's disease and the rare eye disease Leber congenital amaurosis.
Did you know?
- ...that Australian pilot and nurse Robin Miller (monument pictured) borrowed money to buy a Cessna 182 and flew to remote outback areas in Western Australia to vaccinate Indigenous Australian children against polio, becoming known as the "Sugarbird Lady"?
- ...that the Mokola virus is a relative of the rabies virus and was first isolated in tree shrews?
- ...that in 1995–97, a Centers for Disease Control program identified over a hundred patients likely suffering from life-threatening infections new in the history of emerging infectious diseases?
- ...that Batheay District in Kampong Cham Province, Cambodia, was the site of an outbreak of the deadly H5N1 virus in 2006?
- ...that Fig Trees, an operatic documentary about AIDS activism, is narrated by a singing albino squirrel?
Selected biography
Rosalind Elsie Franklin (25 July 1920 – 16 April 1958) was a British biophysicist and X-ray crystallographer who made critical contributions to the understanding of the fine molecular structures of DNA, RNA and viruses.
Franklin led pioneering research on the structure of tobacco mosaic virus (TMV), a rod-like RNA virus, using X-ray crystallography. She first showed that, contrary to contemporary opinion, TMV virus particles were all of the same length. With Kenneth Holmes, she showed the virus's coat is composed of protein molecules arranged in helices. She designed and built a model of the virus to be exhibited at the 1958 World's Fair. She speculated that the virus is hollow, and correctly hypothesized that the RNA of TMV is single-stranded. Her work, together with that of Donald Caspar, revealed that the viral RNA is wound along the inner surface of the hollow virus. Her laboratory, which also included Aaron Klug, studied other plant viruses, including turnip yellow mosaic virus and viruses infecting potato, tomato and pea. Franklin also worked on icosahedral animal viruses, including poliovirus.
Franklin is commemorated in the Rosalind Franklin University of Medicine and Science.
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
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Influenza vaccines include live attenuated and inactivated forms. Inactivated vaccines contain three or four different viral strains selected by the World Health Organization to cover influenza A H1N1 and H3N2, as well as influenza B, and are usually administered by intramuscular injection. The live attenuated influenza vaccine contains a cold-adapted strain and is given as a nasal spray. Most influenza vaccine strains are cultivated in fertilised chicken eggs (pictured), a technique developed in the 1950s; others are grown in cell cultures, and some vaccines contain recombinant proteins. Annual vaccination is recommended for high-risk groups and, in some countries, for all those over six months. As the influenza virus changes rapidly by antigenic drift, new versions of the vaccine are developed twice a year, which differ in effectiveness depending on how well they match the circulating strains. Despite considerable research effort for decades, no effective universal influenza vaccine has been identified. A 2018 meta-analysis found that vaccination in healthy adults decreased confirmed cases of influenza from about 2.4% to 1.1%. However, the effectiveness is uncertain in those over 65 years old, one of the groups at highest risk of serious complications.
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