Viruses are small
infectious agents that can replicate only inside the living
cells of an organism. Viruses infect all forms of life, including
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
RNA, wrapped in a
protein coat called the
capsid; some viruses also have an outer
envelope. The capsid can take simple
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
The origins of viruses are unclear: some may have
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
inflammation of the
gastrointestinal tract involving both the
small intestine, which results in
vomiting, and sometimes
abdominal pain. It is usually caused by a
virus: most commonly
norovirus, but also
astrovirus. Other major infectious causes include
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
oral rehydration therapy. A
rotavirus vaccine is available.
In the news
Map showing the prevalence of SARS-CoV-2 cases; black: highest prevalence; dark red to pink: decreasing prevalence; grey: no recorded cases or no data
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.
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.
14 February: Seven cases of
Ebola virus disease are reported in
7 February: A case of Ebola virus disease is detected in
North Kivu Province of the
Democratic Republic of the Congo.
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.
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.
18 November: The outbreak of
Ebola virus disease in
Democratic Republic of the Congo, which started in June, has been declared over; a total of 130 cases were recorded, with 55 deaths.
1976 Zaire Ebola virus outbreak was one of
the first two recorded outbreaks of
the disease. The causative agent was identified as
a novel virus, named for the region's
Ebola River. The first identified case, in August, worked in the school in
Yambuku, a small rural village in
Mongala District, north
Zaire. He had been treated for suspected
malaria at the Yambuku Mission Hospital, which is now thought to have spread the virus by giving vitamin injections with inadequately sterilised needles, particularly to women attending prenatal clinics. Unsafe burial practices also spread the virus.
The outbreak was contained by quarantining local villages, sterilising medical equipment and providing protective clothing to medical personnel, and was over by early November. A total of 318 cases was recorded, of whom 280 died, an 88%
case fatality rate. An earlier outbreak in June–November in
Sudan, was initially thought to be linked, but was shown to have been caused by
a different species of Ebola virus.
||An inefficient virus kills its host. A clever virus stays with it.
Tobacco mosaic virus (TMV) is an
RNA virus in the
Virgaviridae family that infects a wide range of plants, including
pepper, other members of the
Solanaceae family, and
cucumber. The rod-shaped virus particle is around 300
nm long and 18 nm in diameter, and consists of a
capsid made from 2130 copies of a single coat
protein, which is wrapped around a
genome of around 6400 bases. The coat protein and RNA can
self-assemble to produce infectious virus.
Infection often causes characteristic patterns, such as "
mosaic"-like mottling and discoloration on the leaves, but is almost symptomless in some host species. TMV causes an economically important disease in tobacco plants. Transmission is frequently by human handling, and prevention of infection involves destroying infected plants, hand washing and
crop rotation to avoid contaminated soil. TMV is one of the most stable viruses known. The fact that it does not infect animals and can readily be produced in
gramme amounts has led to its use in numerous pioneering studies in
structural biology. TMV was the first virus to be discovered and the first to be crystallised.
Did you know?
Jonas Edward Salk (28 October 1914 – 23 June 1995) was an American
medical researcher and
virologist, best known for developing the first successful
Unlike most other researchers, Salk focused on creating an inactivated or "killed" virus
vaccine, for safety reasons. The vaccine he developed combines three strains of wild-type
poliovirus, inactivated with
formalin. The field trial that tested its safety and efficacy in 1954 was one of the largest carried out to date, with vaccine being administered to over 440,000 children. When the trial's success was announced, Salk was hailed as a miracle worker and national hero. A little over two years later, 100 million doses of the vaccine had been distributed throughout the US, with few reported adverse effects. An inactivated vaccine based on the Salk vaccine is the mainstay of
polio control in many developed countries.
Salk also researched vaccines against
HIV. In 1960, he founded the
Salk Institute for Biological Studies research centre in
In this month
Nevirapine (also Viramune) is an
antiretroviral drug used in the treatment of
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
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.