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".
Influenza, or flu, is an infectious disease caused by some orthomyxoviruses, that affects birds and some mammals including humans, horses and pigs. Influenza is predominantly transmitted through the air by coughs or sneezes, creating aerosols containing the virus. It can also be transmitted by contact with bird droppings or nasal secretions, or by touching contaminated surfaces. As the virus can be inactivated by soap, frequent hand washing reduces the risk of infection. Around a third of cases show no symptoms. The most common symptoms include fever, runny nose, sore throat, muscle pains, headache, cough and fatigue. Influenza is occasionally associated with nausea and vomiting, particularly in children. Pneumonia is a rare complication which can be life-threatening.
Influenza spreads around the world in seasonal epidemics, resulting in about 3–5 million cases of severe illness annually, and about 250,000–500,000 deaths, mainly in the young, the old and those with other health problems. Annual influenza vaccinations are recommended for those at high risk. Sporadic influenza pandemics have been recorded since at least the 16th century. The Spanish flu pandemic of 1918–20 is estimated to have killed 50–100 million people.
In the news
Map showing the distribution of coronavirus cases; black: highest incidence; dark red to pink: decreasing incidence; grey: no recorded cases
4 April: The ongoing pandemic of a novel coronavirus is accelerating rapidly; more than a million confirmed cases, including more than 57,000 deaths, have been documented globally since the outbreak began in December 2019. WHO 1, 2
27 March: An international, randomised, non-blinded, clinical trial organised by the World Health Organization of four potential treatments for COVID-19 – remdesivir; chloroquine or hydroxychloroquine; lopinavir/ritonavir; or lopinavir/ritonavir plus interferon-beta – is about to start enrolling patients. Science, WHO
16 March: A phase I clinical trial of a messenger RNA-based vaccine candidate for the novel coronavirus begins in Seattle. NIH
11 March: The World Health Organization describes the ongoing outbreak of respiratory disease caused by a novel coronavirus as a pandemic. WHO
10 March: A patient with apparent clearance of HIV after stem-cell therapy continues to have no viable virus detectable in blood or other reservoirs after 30 months without antiretroviral treatment. Lancet
9 March: No new cases have been recorded in three weeks in the ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo; as of 3 March there had been a total of 3444 cases, including 2264 deaths, since the outbreak began in August 2018. WHO 1, 2
False-coloured electron micrograph of novel coronavirus
12 February: The ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo remains a Public Health Emergency of International Concern, according to the World Health Organization. WHO 1
7 February: Chinese scientists announce that novel coronavirus (2019-nCoV) is 99% identical to a coronavirus isolated from pangolins, suggesting these animals might be an intermediate host. Nature
5 February: A study of 2658 samples from 38 different types of cancer found that 16% were associated with a virus, higher than previous estimates, but did not identify any new candidate tumour viruses. Nat Genet
4 February: Over 2500 putative circular DNA virus genomes are catalogued from metagenomic surveys of human and animal samples, including over 600 dissimilar to existing virus groups. eLife, Science
3 February: The US National Institute of Allergy and Infectious Diseases stops the South African HVTN 702 Phase IIb/III clinical trial of an investigational HIV vaccine early, after the vaccine failed to prevent HIV infection. NIH
The Hershey–Chase experiments were conducted by Alfred Hershey and Martha Chase in 1952 using the T2 bacteriophage (pictured), which is composed of DNA wrapped in a protein shell. Hershey and Chase labelled either the phage DNA using radioactive phosphorus-32 or the protein using radioactive sulphur-35. They allowed the radiolabelled phages to infect unlabelled bacteria, and then agitated in a blender and centrifuged to separate material remaining outside the bacterial cells. The majority of the 32P-labelled DNA entered the host bacterial cell, while all the 35S-labelled protein remained outside. Hershey and Chase also showed that the phage DNA is inserted into the bacteria shortly after the virus attaches to its host.
DNA had been known since 1869, but in 1952 many scientists believed that proteins carried the information for inheritance. Proteins appeared more complex, while DNA was thought to be an inert molecule used for phosphorus storage. These experiments built on earlier research on transformation in bacteria and helped to confirm that DNA, not protein, was the genetic material. Hershey shared the 1969 Nobel Prize in Physiology or Medicine for the research.
The 2001 foot-and-mouth outbreak included 2,000 cases of the disease in cattle and sheep across the UK. The source was a Northumberland farm where pigs had been fed infected meat that had not been adequately sterilised. The initial cases were reported in February. The disease was concentrated in western and northern England, southern Scotland and Wales, with Cumbria being the worst-affected area. A small outbreak occurred in the Netherlands, and there were a few cases elsewhere in Europe.
The UK outbreak was controlled by the beginning of October. Control measures included stopping livestock movement and slaughtering over 6 million cows and sheep. Public access to farmland and moorland was also restricted (pictured), greatly reducing tourism in affected areas, particularly in the Lake District. Vaccination was used in the Netherlands, but not in the UK due to concerns that vaccinated livestock could not be exported. The outbreak cost an estimated £8 billion in the UK.
||There are more viruses on Earth than stars in the universe.
Tobacco mosaic virus (TMV) is an RNA virus in the Virgaviridae family that infects a wide range of plants, including tobacco, tomato, 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 helical capsid made from 2130 copies of a single coat protein, which is wrapped around a positive-sense single-stranded RNA 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 virology and structural biology. TMV was the first virus to be discovered and the first to be crystallised.
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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.