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".
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Cervical cancer is a tumour of the cervix, the part of the uterus that adjoins the vagina in the female reproductive tract. Certain types of human papillomavirus (HPV) are implicated in more than 90% of these cancers, although the great majority of HPV infections of the cervix are not associated with cancer. HPV is transmitted by vaginal sex, infecting cervical epithelial cells. In a minority of cases, infection persists for years, and pre-cancerous changes called cervical intraepithelial neoplasia can develop, which sometimes progress to cancer. Squamous cell carcinomas are the most common. HPV infection is considered essential for nearly all forms of cervical cancer to develop, but other risk factors are also involved, including smoking, HIV infection and other forms of immune suppression, and long-term use of oral contraceptives.
Cervical cancer is the fourth most common cancer in women worldwide. It can be detected by screening; screening every 3–5 years, with appropriate follow-up, can reduce cancer incidence by up to 80%. HPV vaccines protect against high-risk virus strains, and can prevent up to 90% of cervical cancers. Where screening and vaccination are not available, cervical cancer has substantial mortality; worldwide, an estimated 569,000 cases and 311,000 deaths occurred in 2018, with around 85% of cases being in developing countries.
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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
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Bats host a diverse array of viruses, including all seven types described by the Baltimore classification system. The most common viruses known to infect bats are coronaviruses. Bats harbour many viruses that are zoonotic, or capable of infecting humans, including rabies virus, SARS-CoV, SARS-CoV-2, MERS-CoV, Nipah virus, Hendra virus and Marburg virus (hosted by the Egyptian fruit bat; pictured), and some bat-borne viruses are considered important emerging viruses. Bats may also play a role in the ecology of the Ebola virus. Most zoonotic bat viruses are transmitted by direct contact with infected bat fluids such as urine, guano and saliva, or through contact with an infected intermediate host; transmission of rabies from bats to humans usually occurs via biting. Butchering or consuming bat meat could potentially lead to viral transmission.
Bats rarely become ill from viral infections, and rabies is the only viral disease known to kill them. They might be more tolerant of infection than other mammals. Their immune systems differ from those of other mammals in their lack of several inflammasomes, which activate the body's inflammatory response, as well as a dampened stimulator of interferon genes response, which helps to control the host response to pathogens.
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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.
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| “ | Viruses are living chemicals... | ” |
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
Hepatitis delta virus or hepatitis D virus (HDV) is a small virusoid, the sole member of the Deltavirus genus. It infects humans. A subviral satellite, it can only replicate in the presence of a hepatitis B (HBV) helper virus. The spherical virion is 36 nm in diameter, with an envelope containing three HBV proteins. The single-stranded, negative-sense, circular RNA genome of 1679 nucleotides is smaller than that of any known animal virus. It has an unusual base composition for an entity that infects animals, and is extensively bound to itself to form a partially double-stranded, rod-shaped structure. These features have led to suggestions that HDV might be related to viroids, small unencapsidated circular RNAs that infect plants. Unlike viroids, HDV encodes a protein, hepatitis D antigen.
Both HDV and HBV enter liver cells using the sodium/bile acid cotransporter as their receptor. They are mainly transmitted via injecting drug use and blood products. More than 15 million people are infected with both viruses, which is associated with a greater risk of liver complications than HBV infection alone. Around one in five jointly infected patients die. The HBV vaccine protects against HDV.
Did you know?
- ...that during the Boonah crisis in October 1918, 31 people died of the Spanish flu aboard the Australian troopship the HMAT Boonah (pictured)?
- ...that Deng Hongkui and Chen Hu were the first to use CRISPR gene editing on an HIV patient?
- ...that adenovirus serotype 14 is an emerging virus, related to the common cold, that has recently caused 10 deaths in the United States, including at least one healthy young adult?
- ...that the phage group was an informal network of biologists centered around Max Delbrück that contributed heavily to the origins of molecular biology?
- ...that polio eradication efforts in Pakistan were hampered by a vaccination campaign conducted by the CIA as cover in the search for Osama bin Laden?
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Peter Piot (born 17 February 1949) is a Belgian virologist and public health specialist, known for his work on Ebola virus and HIV.
During the first outbreak of Ebola in Yambuku, Zaire in 1976, Piot was one of a team that discovered the filovirus in a blood sample. He and his colleagues travelled to Zaire to help to control the outbreak, and showed that the virus is transmitted via blood and during preparation of bodies for burial. He advised WHO during the West African Ebola epidemic of 2014–16.
In the 1980s, Piot participated in collaborative projects in Burundi, Côte d'Ivoire, Kenya, Tanzania and Zaire, including Project SIDA in Kinshasa, the first international project on AIDS in Africa, which provided the foundations for understanding HIV infection in that continent. He was the founding director of UNAIDS, and has served as president of the International AIDS Society and assistant director of the WHO Global HIV/AIDS Programme. As of 2020, he directs the London School of Hygiene & Tropical Medicine.
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
Several human papillomavirus (HPV) vaccines, including Cervarix and Gardasil, have been approved to protect against infections with particular types of HPV, associated with cervical and other cancers. All vaccines protect against the high-risk HPV types 16 and 18. Gardasil is a quadrivalent vaccine that additionally protects against low-risk HPV-6 and -11, which are associated with most cases of genital warts. A second-generation nine-valent Gardasil vaccine protects against five additional high-risk HPV types. It is estimated that the vaccines may prevent 70% of cervical cancer, 80% of anal cancer, 60% of vaginal cancer, 40% of vulvar cancer and possibly some oropharyngeal cancers. Protection lasts for at least 8–9 years. Some advocate giving Gardasil to men and boys with the primary aim of protecting their female sexual partners; others consider vaccinating only women and girls to be more cost effective. The licensed vaccines are subunit vaccines, containing only the L1 capsid protein of the virus, which self-assembles into virus-like particles. They are not effective in people already infected with HPV. Research is ongoing into therapeutic HPV vaccines including the viral oncoproteins, E6 and E7, but none has yet been licensed.
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