Portal:Viruses
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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".
Measles is a disease that only affects humans caused by the measles virus, an RNA virus in the Paramyxoviridae family. It is highly contagious, with transmission occurring via the respiratory route or by contact with secretions. Symptoms generally develop 10–12 days after exposure and last 7–10 days; they include high fever, cough, rhinitis and conjunctivitis, white Koplik's spots inside the mouth and a generalised red maculopapular rash. Complications including diarrhoea, otitis media and pneumonia are relatively common; more rarely seizures, encephalitis, croup, corneal ulceration and blindness can occur. The risk of death is usually around 0.2%, but may be as high as 10–28% in areas with high levels of malnutrition and poor healthcare.
Measles was first described by Rhazes (860–932). The disease is estimated to have killed around 200 million people between 1855 and 2005. It affects about 20 million people a year, primarily in the developing areas of Africa and Asia, and is one of the leading vaccine-preventable disease causes of death. No antiviral drug is licensed. An effective measles vaccine is available, but uptake has been reduced by anti-vaccination campaigns, particularly the fraudulent claim that the MMR vaccine might be associated with autism. Rates of disease and deaths increased from 2017 to 2019, attributed to a decrease in immunisation.
Culex species mosquitoes transmit West Nile virus. Elimination of the stagnant water pools where the mosquitoes breed, together with other mosquito control measures, is key to preventing disease.
Credit: James Gathany (28 February 2006)
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
Infectious diseases are symptomatic diseases of an individual host resulting from the infection and replication of pathogens, including viruses, prions, bacteria, fungi, protozoa and multicellular parasites. Infectious diseases were responsible for 17% of human deaths globally in 2013, with HIV, measles and influenza being among the most significant viral causes of death.
Infectious pathogens must enter, survive and multiply within the host, and spread to fresh hosts. Relatively few microorganisms cause disease in healthy individuals, and infectious disease results from the interplay between these rare pathogens and the host's defences. Infection does not usually result in the host's death. The pathogen is generally cleared from the body by the host's immune system, although persistent infection occurs with several viruses. Transmission can occur by physical contact, contaminated food, water or objects, body fluids, airborne inhalation or via vectors, such as the mosquito (pictured). Diagnosis sometimes involves identifying the pathogen; techniques include culture, microscopy, immunoassays and PCR-based diagnostics.
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.
| “ | ...in a flash I had understood: what caused my clear spots was, in fact, an invisible microbe, a filterable virus, but a virus parasitic on bacteria. | ” |
—Félix d'Herelle on the discovery of bacteriophages
Viruses & Subviral agents: bat virome • elephant endotheliotropic herpesvirus • HIV • introduction to viruses
Diseases: colony collapse disorder • common cold • croup • dengue fever
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
Methodology: metagenomics
Social & Media: And the Band Played On • Contagion • "Flu Season" • Frank's Cock
People: Brownie Mary • Macfarlane Burnet
Henipaviruses are a genus of RNA viruses in the Paramyxoviridae family. The variably shaped, 40–600 nm diameter, enveloped capsid contains a single-stranded, negative-sense RNA genome of 18.2 kb, with six genes. The cellular receptor is in the ephrin family. The natural hosts are predominantly bats, mainly the Pteropus genus of megabats (flying foxes) and some microbats. Bats infected with Hendra virus develop viraemia and shed virus in urine, faeces and saliva for around a week, but show no signs of disease. Henipaviruses can also infect humans and livestock, causing severe disease with high mortality, making the group a zoonootic disease. Transmission to humans sometimes occurs via an intermediate domestic animal host.
The first henipavirus, Hendra virus, was discovered in 1994 as the cause of an outbreak in horses in Brisbane, Australia. Nipah virus was identified a few years later in Malaysia as the cause of an outbreak in pigs. Three further species have since been recognised: Cedar and Kumasi viruses in bats, and Mòjiāng virus in rodents. Their emergence as human pathogens has been linked to increased contact between bats and humans. Human disease has been confined to Australia and Asia, but members of the genus have also been found in African bats. A veterinary vaccine against Hendra virus is available but no human vaccine has been licensed.
- ...that Franquet's Epauletted Fruit Bat (pictured) is one of three fruit-eating bats found to be a reservoir for Ebola virus in the wild?
- ...that 95% of adults have been infected by human herpesvirus 7, a virus that can cause influenza-like illness and seizures but normally causes no symptoms?
- ...that James Jurin used statistical studies to show the probability of death from smallpox vaccine was significantly less than from smallpox?
- ...that the log-Cauchy distribution has been proposed as a model for the progression of HIV in individuals?
- ...that Corona, named after Corona, is fighting corona?
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.
1 September 1910: Peyton Rous shows that a sarcoma of chickens, subsequently associated with Rous sarcoma virus, is transmissible
3 September 1917: Discovery of bacteriophage of Shigella by Félix d'Herelle
8 September 1976: Death of Mabalo Lokela, the first known case of Ebola virus
8 September 2015: Discovery of giant virus Mollivirus sibericum in Siberian permafrost
11 September 1978: Janet Parker was the last person to die of smallpox
12 September 1957: Interferon discovered by Alick Isaacs and Jean Lindenmann
12 September 1985: Structure of human rhinovirus 14 (pictured) solved by Michael Rossmann and colleagues, the first atomic-level structure of an animal virus
17 September 1999: Jesse Gelsinger died in a clinical trial of gene therapy using an adenovirus vector, the first known death due to gene therapy
20 September 2015: Wild poliovirus type 2 declared eradicated
26 September 1997: Combivir (zidovudine/lamivudine) approved; first combination antiretroviral
27 September 1985: Structure of poliovirus solved by Jim Hogle and colleagues
28 September 2007: First World Rabies Day is held
Oseltamivir (also Tamiflu) is an oral antiviral drug against influenza (flu). It was the second inhibitor of the viral neuraminidase to be developed, after zanamivir, and the first to be taken as an oral tablet. It was originally synthesised from shikimic acid extracted from the star anise plant. Oseltamivir is a prodrug that requires metabolism in the liver to the active form, oseltamivir carboxylate. This binds at the active site of the neuraminidase enzyme, preventing it from cleaving sialic acid to release the virus particle from the host cell. Oseltamivir can reduce the duration of influenza symptoms by 0.5–1 days. Debate is ongoing about whether it also reduces the risk of complications, such as pneumonia. Nausea and vomiting are the main adverse events. Resistance to oseltamivir has been observed in some strains of influenza virus, especially H1N1 strains.
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