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".
Gastroenteritis is an infectious disease of the gastrointestinal tract involving both the stomach and small intestine, which results in diarrhoea and vomiting, and sometimes abdominal pain. It can be caused by several types of virus: most commonly rotavirus and norovirus, but also adenovirus and astrovirus. Other major causes include Campylobacter, Escherichia coli, Vibrio cholerae and some other bacteria, as well as parasites. 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 can decrease incidence by as much as 30%. An estimated 3–5 billion cases of gastroenteritis occur globally each year, mainly among children and people in developing countries, resulting in 1.4 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.
Vaccination or immunisation is the administration of antigenic material (a vaccine) to stimulate an individual's immune system to develop adaptive immunity to a virus or other pathogen. The active agent of a vaccine may be intact but inactivated (non-infective) or attenuated (with reduced infectivity) forms of the pathogen, or purified components that have been found to be highly immunogenic, such as viral envelope proteins. Smallpox was the first disease for which a vaccine was produced, by Edward Jenner in 1796.
Vaccination is the most effective method of preventing infectious diseases and can also ameliorate the symptoms of infection. Widespread immunity due to mass vaccination campaigns is largely responsible for the worldwide eradication of smallpox and the restriction of diseases such as polio and measles from much of the world. Vaccination efforts have been met with some controversy since their inception, on scientific, ethical, political, medical safety, and religious grounds.
In the news
18 January: The Coalition for Epidemic Preparedness Innovations commits $460 million to fast-tracking the development of vaccines against MERS coronavirus, Lassa virus and Nipah virus. BBC
18 November: WHO declares that Zika virus transmission and associated conditions (virus pictured) are a long-term situation which no longer qualifies as a Public Health Emergency of International Concern. WHO
4 October: Capsid protein assemblies of beak and feather disease virus, which infects endangered parrot species (infected bird pictured), are visualised by X-ray crystallography. Nat Commun
29 September: Zika virus is shown to infect the neural crest cells that develop into the cranium, driving cell death of neural progenitor cells by their impaired cytokine signalling. Cell Host Microbe
28 September: Ranaviruses (pictured), which cause severe disease in wild amphibians, are found to be spread in the UK by human activities. Proc R Soc London B
27 September: Rift Valley fever virus infection during pregnancy substantially increased the risk of miscarriage in a cross-sectional study in Sudan. Lancet Global Health
27 September: WHO declares measles to have been eliminated from North and South America. PAHO/WHO
26 September: Broadly neutralising antibodies to HIV are found in 239 HIV+ participants in the Swiss Cohort Study, with a significantly higher rate among black people. Nat Med
15 September: In a novel mouse model of hepatitis A virus infection, acute liver inflammation is found to be caused by hepatocyte apoptosis as an intrinsic response to infection. Science
13 September: A broadly neutralising antibody, which binds to the haemagglutinin stem and recognises most influenza A strains, is shown to be produced by human memory B cells. Nat Commun
2 September: Modelling suggests that the recently approved Sanofi-Pasteur vaccine may increase the frequency of severe dengue symptoms where viral transmission is low. Science
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.
||Viruses are living chemicals...
Rotavirus is a genus of double-stranded RNA viruses in the family Reoviridae. There are five species A–E; rotavirus A, the most common, causes over 90% of infections in humans. Rotavirus also infects animals, including livestock. The virus is transmitted by the faecal–oral route, with fewer than 100 virus particles being required for infection. Rotaviruses are stable in the environment and normal sanitary measures fail to protect against them. Effective rotavirus vaccines are the main prevention method.
The virus infects and damages the enterocytes lining the small intestine, causing gastroenteritis (sometimes referred to as "stomach flu," although the virus is not related to influenza). A viral toxin is responsible for some of the pathology. Rotavirus is the most common cause of severe diarrhoea among infants and young children. Almost every child worldwide has been infected with rotavirus at least once by the age of five. Over 500,000 children under five die from rotavirus infection each year and almost two million more become severely ill. Immunity develops with repeated infections and adults are rarely affected.
Did you know?
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.
In this month
A selection of recent articles of interest include: