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
HIV-1 budding from lymphocytes in culture. HIV establishes a latent infection in several types of immune cell and causes profound immunodeficiency.
Credit: C. Goldsmith (1984)
In the news
27 November: In the ongoing pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), more than 60 million confirmed cases, including more than 1.4 million deaths, have been documented globally since the outbreak began in December 2019. 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
16 November: Interim unpublished results from an ongoing Phase III trial in around 30,000 participants suggest that an mRNA vaccine from Moderna targeting the SARS-CoV-2 S glycoprotein is at least 94% effective at preventing COVID-19 disease after two doses. Nature
9 November: Interim unpublished results from an ongoing Phase III trial in 43,538 participants suggest that an mRNA vaccine from Pfizer/BioNTech targeting the SARS-CoV-2 S glycoprotein is at least 90% effective at preventing COVID-19 disease after two doses. Nature
5 November: A cluster of COVID-19 cases associated with mink farming, in which the causative SARS-CoV-2 strain harbours a novel combination of mutations and appears less susceptible to neutralising antibodies, are reported from North Jutland, Denmark; the news triggers the slaughter of all farmed mink across the country. WHO
29 October: The Howard Hughes Medical Institute announces a donation to the Henrietta Lacks Foundation, intended as reparation for the harvesting of cervical cancer cells from Lacks in 1951, without her consent; the cells were subsequently used to create the HeLa cell line. Nature
22 October: The US FDA approves the repurposed antiviral remdesivir for the treatment of SARS-CoV-2 infection; the decision has been criticised owing to the drug's apparent lack of benefit in as-yet-unpublished interim results from the World Health Organization's (WHO) Solidarity trial. Science
14 October: A combination of three monoclonal antibodies, atoltivimab, maftivimab and odesivimab-ebgn, is licensed by the US FDA as the first treatment for Ebola virus disease. FDA
25 August: Africa is declared free from wild poliovirus, with the last case having been detected in Nigeria in 2016; vaccine-derived polio cases continue to be recorded in the region. Nature
20 August: The International Committee on Taxonomy of Viruses creates three new realms for DNA viruses – Duplodnaviria, for double-stranded DNA (dsDNA) viruses with HK97-fold capsid proteins, including tailed bacteriophages and herpesviruses; Varidnaviria for dsDNA viruses with jelly-roll capsid proteins, such as adenoviruses and poxviruses; and Monodnaviria for most single-stranded DNA viruses, including papillomaviruses – and also extends Riboviria to include RNA viruses that replicate using reverse transcriptase. ICTV
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.
Selected outbreak
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In the severe acute respiratory syndrome (SARS) outbreak, the first cases of the newly emerged SARS coronavirus were reported in November 2002 from the Chinese Guangdong province. The virus soon spread across Asia, with China, Hong Kong, Taiwan and Singapore being the worst affected countries; a secondary outbreak occurred in Canada. The rapid initial spread of the outbreak has been in part attributed to China's slow response to the early cases. Over 8,000 people were infected, with a case fatality rate of 11%. Those over 65 years had a much higher mortality rate, greater than 55%. The outbreak was contained by July 2003, and no cases have been reported since 2004.
At the time of the outbreak, the immediate source of SARS coronavirus was thought to have been the masked palm civet (Paguma larvata; pictured), which was sold as food in Guangdong markets. The virus was also found in raccoon dogs, ferret badgers and domestic cats. More recent research has suggested that the natural reservoir could be horseshoe bats.
Selected quotation
| “ | Viruses are living chemicals... | ” |
Selected virus
Cauliflower mosaic virus (CaMV) is a plant pararetrovirus in the Caulimoviridae family, which has similarities with hepadnaviruses such as hepatitis B virus. It predominantly infects members of the Brassicaceae (cabbage) family, including cauliflower and turnip; some strains can also infect Datura and Nicotiana species of the Solanaceae family. It is transmitted by aphid vectors, such as Myzus persicae. Symptoms include a mottled leaf pattern called "mosaic", necrotic lesions on the surface of infected leaves, stunted growth and deformation of the overall plant structure.
Although the viral genome is double-stranded DNA, the virus replicates via reverse transcription like a retrovirus. The icosahedral virion is 52 nm in diameter, and is built from 420 capsid protein subunits. The circular 8 kb genome encodes seven proteins, including a movement protein, which facilitates viral movement to neighbouring cells, and an insect transmission factor, which recognises a protein receptor at the tip of the aphid mouthparts (pictured). CaMV has several ways of evading the host defensive responses, which include interrupting salicylic acid-dependent signalling and decoying host silencing machinery. The virus has a strong constitutive (always on) promoter, CaMV 35S, which is widely used in plant genetic engineering.
Did you know?
- ...that Hungarian veterinarian József Marek, the first to identify Marek's disease (symptom pictured; right), also developed a remedy for liver fluke in cattle and a nasogastric tube for treating horse colic?
- ...that because photomicrographs of Longan witches broom-associated virus were not published, the virus was not initially accepted as the cause of the eponymous disease?
- ...that Shetlander Johnnie Notions, a physician with no formal medical background, developed a smallpox inoculation that successfully immunised thousands of people before Jenner's vaccine was available?
- ...that the Kivu Ebola epidemic was declared a Public Health Emergency of International Concern on 17 July 2019?
- ...that Larry Kramer frequently invoked the AIDS–Holocaust metaphor and compared Reagan Administration officials to Nazi war criminals?
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
Nevirapine (also Viramune) is an antiretroviral drug used in the treatment of HIV/AIDS caused by HIV-1. It was the first non-nucleoside reverse transcriptase inhibitor to be licensed, which occurred in 1996. Like nucleoside inhibitors, nevirapine inhibits HIV's reverse transcriptase enzyme, which copies the viral RNA into DNA and is essential for its replication. Unlike nucleoside inhibitors, it binds not in the enzyme's active site but in a nearby hydrophobic pocket, causing a conformational change in the enzyme that prevents it from functioning. Mutations in the pocket generate resistance to nevirapine, which develops rapidly unless viral replication is completely suppressed. The drug is therefore only used together with other anti-HIV drugs in combination therapy. The HIV-2 reverse transcriptase has a different pocket structure, rendering it inherently resistant to nevirapine and other first-generation NNRTIs. A single dose of nevirapine is a cost-effective way to reduce mother-to-child transmission of HIV, and has been recommended by the World Health Organization for use in resource-poor settings. Other protocols are recommended in the United States. Rash is the most common adverse event associated with the drug.
Subcategories
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
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