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The capsid of SV40, an icosahedral virus

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 5,000 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

Hand washing with soap is a protective measure against gastroenteritis

Gastroenteritis is an inflammation of the gastrointestinal tract involving both the stomach and small intestine, which results in diarrhoea and vomiting, and sometimes abdominal pain. It is usually caused by a virus: most commonly rotavirus and norovirus, but also adenovirus and astrovirus. Other major infectious causes include Campylobacter, Escherichia coli, Vibrio cholerae and some other bacteria, as well as protozoa. Viruses, particularly rotavirus, cause about 70% of gastroenteritis episodes in children, while norovirus is the leading cause of gastroenteritis among adults in America, causing over 90% of outbreaks.

Transmission can be from consumption of improperly prepared foods or contaminated water, or by close contact with infectious individuals. Good sanitation practices and a convenient supply of uncontaminated water are important for reducing infection. Personal measures such as hand washing with soap can decrease incidence by as much as 30%. An estimated 2 billion cases of gastroenteritis occurred globally in 2015, mainly among children and people in developing countries, resulting in 1.3 million deaths. Gastroenteritis is usually an acute and self-limiting disease that does not require medication; the main treatment is rehydration using oral rehydration therapy. A rotavirus vaccine is available.

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Egyptian stele believed to show a poliomyelitis survivor

This 18th Dynasty Egyptian stele, believed to show a priest with poliomyelitis-associated deformity, is one of the earliest records of a viral disease.

Credit: Unknown (1580–1350 BC)

Selected article

Baltimore classification

Virus classification is the process of naming viruses and placing them into a taxonomic system. Viruses do not fit neatly into the biological classification system used for cellular organisms. They are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms and the type of disease they cause.

Two schemes are in common use. The Baltimore classification (pictured), proposed in 1971 by David Baltimore, places viruses into seven groups (I–VII) based on their nucleic acid type, number of strands and sense, as well as the method the virus uses to generate mRNA. The International Committee on Taxonomy of Viruses, established in the early 1970s, classifies viruses into groupings similar to those used for cellular organisms, which reflect viruses thought to have a common ancestor. As of 2019, one phylum, 6 classes, 14 orders, 143 families, 846 genera and 4,958 species of viruses have been defined. The majority of virus families have not yet been assigned to an order.

In the news

Diagram of African swine fever virus

2 July: In the ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo (DRC) – now the second-largest Ebola outbreak in history – new cases remain stable, with 285 reported in the past 3 weeks, and a total of 2372 cases, including 1602 deaths, since the outbreak began in August 2018. WHO 1, 2

26 June: A meta-analysis of vaccinating girls and women against human papillomavirus including 60 million vaccinees finds that diagnoses of high-grade cervical intraepithelial neoplasia and cervical cancer in women and of anogenital warts in both women and men have reduced in frequency. Lancet

14 June: The WHO Emergency Committee declares that the ongoing DRC Ebola virus outbreak fails to meet the criteria for a Public Health Emergency of International Concern, despite the spread to Uganda. WHO

3 May: In the ongoing Rift Valley fever outbreak in the Mayotte Islands in the Comoro group there have been 129 confirmed cases since the outbreak started in November 2018. WHO

2 May: A European observational study in 972 gay male couples finds no HIV transmission with unprotected sex where the HIV-positive partner's virus is fully suppressed by antiretroviral therapy. Lancet

25 April: A major outbreak of African swine fever ongoing in pigs in China since August 2018 has caused the loss of at least 40 million pigs, and the virus (pictured) has also been reported elsewhere in Southeast Asia. BBC

15 April: The directors of WHO and UNICEF warn that the more than 110,000 measles cases reported globally in January–March represent a nearly threefold increase over the same period in 2018. CNN

14 April: In the ongoing chikungunya virus outbreak in Congo, 6,149 suspected cases have been reported since the outbreak began in January, with nearly half in Kouilou Department. WHO

29 March: The filamentous bacteriophage Pf is shown to increase the pathogenicity of its bacterial host, Pseudomonas aeruginosa, an important human pathogen, by protecting it from the immune system in a mouse model. Science

12 March: The plant nanovirus, faba bean necrotic stunt virus – which has a segmented (multi-part) genome, with each of the eight segments being packaged separately – is shown to be able to replicate successfully even when its DNA segments do not all enter the same cell. eLife

8 March: The International Committee on Taxonomy of Viruses ratifies an update to virus classification, creating the Riboviria taxon for all RNA viruses at the new rank of realm. ICTV

5 March: Another case of apparent clearance of HIV from an infected patient after stem-cell therapy is reported. Nature

Selected outbreak

Quarantine notices at the East Birmingham Hospital where the first case was initially treated

The last recorded smallpox death occurred during the 1978 smallpox outbreak in Birmingham, UK. The outbreak resulted from accidental exposure to Variola major, probably the Abid strain, from a WHO-funded laboratory, headed by Henry Bedson, at the University of Birmingham Medical School – also the probable source of a 1966 outbreak. Bedson was investigating strains of smallpox known as whitepox, considered a potential threat to the smallpox eradication campaign, then in its final stages. The virus appears to have spread between floors in late July, possibly via ducting, to infect a medical photographer who worked above the laboratory. She showed symptoms in August and died the following month; one of her contacts was also infected but survived. Bedson committed suicide while under quarantine.

A government inquiry into the outbreak criticised the university's safety procedures. Radical changes in UK research practices for handling dangerous pathogens followed, and all known stocks of smallpox virus were concentrated in two laboratories.

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Selected virus

Electron micrograph of Sin Nombre virus, a hantavirus

Hantaviruses (or orthohantaviruses) are a family of RNA viruses in the Bunyavirales order. The enveloped virion is 120–160 nm in diameter and contains a single-stranded, negative-sense RNA genome with three segments. They infect many species of rodents, as well as shrews and moles, without causing disease, and can be transmitted to humans, where they can cause serious disease. Hantaan virus, the first known hantavirus, was discovered in 1976 as the cause of a novel haemorrhagic fever affecting combatants in the Korean War; it can also be caused by other hantaviruses, including Dobrava-Belgrade and Seoul viruses. Some hantaviruses, including Sin Nombre and Bayou, cause a pulmonary syndrome. Others have not yet been associated with human disease.

Unlike other bunyaviruses, hantaviruses are not transmitted by arthropods. Rodents act as the vector, with transmission to humans usually occurring via contact with urine, saliva or faeces, by inhalation of aerosolised excreta or by bite. Little is understood about how hantaviruses cause disease; the main site of viral replication in the body is unknown. Rodent control is important in disease prevention.

Did you know?

Urera baccifera

Selected biography

Thomas Flewett in 1984

Thomas Flewett (29 June 1922 – 12 December 2006) was a British–Irish virologist and an authority on electron microscopy of viruses, best known for his role in the discovery of rotaviruses. After Ruth Bishop and others discovered viruses associated with diarrhoea, Flewett showed that they could be visualised by electron microscopy directly in faeces. He dubbed them "rotaviruses" for their wheel-shaped appearance. His group described the different rotavirus serotypes, and did extensive research on the rotavirus varieties infecting many animals.

Flewett established one of the first English virus laboratories in Birmingham in 1956. In addition to his rotavirus work, he discovered the cause of hand, foot and mouth disease, identified two new species of adenovirus, and co-discovered human torovirus and picobirnaviruses. His other research included influenza, coxsackie A, coxsackie B and hepatitis B viruses.

In this month

Electron micrograph of SARS coronaviruses

7 November 1991: Magic Johnson announced his retirement from basketball because of his infection with HIV

14 November 1957: Kuru, the first human prion disease, described by Daniel Gajdusek and Vincent Zigas

16 November 2002: The first case of severe acute respiratory syndrome (virus pictured) recorded in Guangdong, China

17 November 1995: Lamivudine approved for treatment of HIV

22 November 2013: Simeprevir approved for treatment of chronic hepatitis C virus infection

23 November 1978: Structure of tomato bushy stunt virus solved by Stephen Harrison and colleagues, the first atomic-level structure of a virus

24 November 2007: Outbreak of new Ebola species, Bundibugyo virus

26 November 1898: Martinus Beijerinck coined the term contagium vivum fluidum to describe the agent causing tobacco mosaic disease

Selected intervention

Child receiving the oral polio vaccine

Two polio vaccines are used against the paralytic disease polio. The first, developed by Jonas Salk, consists of inactivated poliovirus. Based on three wild virulent strains, inactivated using formalin, it is administered by injection and is very safe. It confers IgG-mediated immunity, which prevents poliovirus from entering the bloodstream and protects the motor neurons, eliminating the risk of bulbar polio and post-polio syndrome. The second, developed by Albert Sabin, originally consisted of three live virus strains, attenuated by growth in cell culture. Since 2016, only two strains have generally been included. They contain multiple mutations, preventing them from replicating in the nervous system. The Sabin vaccine stimulates both antibodies and cell-mediated immunity, providing longer-lasting immunity than the Salk vaccine. It can be administered orally, making it more suitable for mass vaccination campaigns. In around three cases per million doses, the live vaccine reverts to a virulent form and causes paralysis. Vaccination has reduced the number of wild-type polio cases from around 350,000 in 1988 to just 33 in 2018, and eradicated the disease from most countries.

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