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".
Herpes simplex is caused by herpes simplex virus types 1 and 2 of the Herpesviridae family, with 60–95% of adults being infected with one of the types. Common forms of infection are oral herpes, which can result in cold sores, and genital herpes. Active disease often involves blisters containing infectious virus, although the genital form is frequently asymptomatic. Less common disorders associated with the viruses include herpetic whitlow, herpes gladiatorum, ocular herpes, herpesviral encephalitis and Mollaret's meningitis.
After initial infection, virus particles are transported along sensory nerves to the cell bodies in the ganglion, where they become latent and remain lifelong. Periods of remission alternate with outbreaks of active disease, in which the virus multiplies in the nerve cell and new virus particles are transported along the nerve fibre to the nerve terminals in the skin, where they are released. What causes these recurrences is unclear. Transmission is usually by direct contact with a lesion or with body fluids, and can occur during periods of asymptomatic shedding. Neonatal herpes is possible after transmission from the mother. Barrier protection methods reduce genital herpes risk. No vaccine or cure exists, but antiviral treatment can alleviate symptoms and reduce viral shedding.
ΦX174 is a bacteriophage whose DNA genome size of 5386 nucleotides, among the smallest of DNA viruses, has led to it being the subject of pioneering research in molecular biology.
Credit: Fdardel (21 March 2009)
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. 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
The 2009 flu pandemic was an influenza pandemic first recognised in Mexico City in March 2009 and declared over in August 2010. It involved a novel strain of H1N1 influenza virus with genes from five different viruses, which resulted when a previous triple reassortment of avian, swine and human influenza viruses further combined with a Eurasian swine influenza virus, leading to the term "swine flu" being used for the pandemic. It was the second pandemic to involve an H1N1 strain, the first being the 1918 "Spanish flu" pandemic.
The global infection rate was estimated as 11–21%. This pandemic strain was less lethal than previous ones, killing about 0.01–0.03% of those infected, compared with 2–3% for Spanish flu. Most experts agree that at least 284,500 people died, mainly in Africa and Southeast Asia – comparable with the normal seasonal influenza fatalities of 290,000–650,000 – leading to claims that the World Health Organization had exaggerated the danger.
||A virus is not an individual organism in the ordinary sense of the term, but something which could almost be called a stream of biological patterns.
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.
Did you know?
Edward Jenner (1749–1823) was an English physician and scientist who pioneered the smallpox vaccine, the world's first vaccine. Noting the common observation that milkmaids were generally immune to smallpox, Jenner postulated that the pus in the blisters that milkmaids received from cowpox (a similar but much less virulent disease) protected them from smallpox. In 1796, Jenner tested his hypothesis by inoculating an eight-year-old boy with pus from an infected milkmaid. He subsequently repeatedly challenged the boy with variolous material, then the standard method of immunisation, without inducing disease. He published a paper including 23 cases in 1798. Although several others had previously inoculated subjects with cowpox, Jenner was the first to show that the procedure induced immunity to smallpox. He later successfully popularised cowpox vaccination.
Jenner is often called "the father of immunology", and his work is said to have saved more lives than that of any other individual.
In this month
1 August 1971: The term viroid was coined by Theodor Diener to describe the agent of potato spindle tuber disease
6 August 2007: Maraviroc, first CCR5 receptor antagonist, approved for HIV/AIDS
8 August 2011: UN declared rinderpest eradicated
8 August 2014: WHO declared the Ebola outbreak in West Africa (virus pictured), the most widespread so far, an international public health emergency
18 August 1990: Ryan White Care Act enacted, the largest American federally funded programme for people living with HIV/AIDS
20 August 1780: Start of an outbreak of dengue fever in Philadelphia, USA, which led Benjamin Rush to describe the disease in 1789
26 August 1976: First case of Ebola virus, now the Zaire form
26 August 1998: Fomivirsen, first antisense drug, approved for cytomegalovirus retinitis
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.