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The Viruses Portal

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

Hepatocellular carcinoma, a type of liver cancer, in an autopsy specimen from a person positive for hepatitis C

Hepatitis C is an infectious disease affecting primarily the liver, caused by hepatitis C virus (HCV), an RNA virus of the Flaviviridae family which infects humans and chimpanzees. A "non-A non-B hepatitis" was postulated in the 1970s, and HCV was demonstrated in 1989. HCV is spread primarily by blood-to-blood contact associated with intravenous drug use in the developed world, and with poorly sterilised medical equipment and transfusions in the developing world. In about 80% of those infected, the virus persists in the liver, and around 10–30% of those infected will develop cirrhosis over 30 years. Some people with cirrhosis go on to develop liver failure, liver cancer or other complications.

An estimated 150–200 million people worldwide are infected with HCV. Hepatitis C causes 27% of cirrhosis cases and 25% of hepatocellular carcinoma (a form of liver cancer) worldwide, and is the leading reason for liver transplantation. The standard therapy for persistent infection is a combination of the antiviral drug ribavirin with pegylated interferon, sometimes plus one of the protease inhibitors, boceprevir or telaprevir. Overall, 50–80% of people treated are cured. No vaccine against hepatitis C is available.

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American Federal Art Project poster dating from 1937 about the common cold

The common cold is the most frequent infectious disease. Despite the advice to "consult your physician" no antiviral treatment has been approved, and colds are only rarely associated with serious complications.

Credit: Federal Art Project (1937)

Selected article

Child receiving oral polio vaccine in India

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

False-coloured micrograph of Middle East respiratory syndrome coronavirus

22 June: Vaccine-derived poliovirus type 1 is confirmed to be circulating in Morobe Province, Papua New Guinea, after the country was declared free of the virus in 2000. WHO

16 June: An outbreak of Rift Valley fever has been confirmed in northern Kenya, with 26 human cases mainly in Wajir County, including 6 deaths, as well as widespread deaths and abortions in camels, goats and other livestock. WHO

1 June: In the ongoing outbreak of Nipah virus in Kerala state, south India, there have been 18 confirmed cases with 17 deaths in the Kozhikode and Mallapuram districts. WHO

31 May: In the ongoing outbreak of Middle East respiratory syndrome coronavirus (pictured), 75 cases have been reported in Saudi Arabia since 12 January, including 23 deaths. WHO

23 May: An outbreak of measles is ongoing in the Amazonas and Roraima states of Brazil, with 995 suspected cases, including two deaths. WHO

21 May: Vaccination with rVSV-ZEBOV starts in the Ebola virus outbreak in the Democratic Republic of the Congo; the outbreak has spread from rural Bikoro to the city of Mbandaka, with a total of 58 suspected cases reported, including 27 deaths. WHO

17 May: The sialic acid-bearing cellular receptor for influenza A virus is shown to be the voltage-gated calcium channel, Cav1.2 (pictured). Cell Host & Microbe

Ribbon diagram of Cav1.2

16 May: The cellular receptor for several alphaviruses associated with rheumatic disease, including chikungunya, Mayaro, O'nyong nyong and Ross River viruses, is shown to be the cell adhesion molecule Mxra8. Nature

14 May: IMP-1088, a compound targeting human N-myristoyltransferases NMT1 and NMT2, is shown to prevent three different picornaviruses, rhinovirus, poliovirus and foot-and-mouth disease virus, from assembling capsids in vitro, by inhibiting myristoylation of the picornaviral VP0 protein. Nat Chem

9 May: Hepatitis B virus sequences are recovered from Bronze Age human remains up to 4,500 years old, and the virus is estimated to have evolved 8,600–20,900 years ago, disproving the hypothesis that it originated in the New World and spread to Europe in around the 16th century. Nature

Selected outbreak

Map showing Ebola virus disease cases in Guinea, Liberia and Sierra Leone in December 2014

The West African Ebola epidemic was the most widespread outbreak of the disease to date, and the first to occur outside Sub-Saharan Africa. Beginning in Meliandou in southern Guinea in December 2013, it spread to adjacent Liberia and Sierra Leone, affecting the densely populated cities of Conakry and Monrovia, with minor outbreaks in Mali and Nigeria. The epidemic was under control by late 2015, but occasional cases continued to occur into June of the following year. More than 28,000 suspected cases were reported with more than 11,000 deaths, a case fatality rate of around 58% in hospitalised patients and up to 70% overall. Around 10% of the dead were healthcare workers.

Extreme poverty, a dysfunctional healthcare system, a distrust of government officials after years of armed conflict, local burial customs involving washing the body after death, and a delay in response of several months all contributed to the failure to contain the epidemic.

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Selected virus

Electron micrograph of Acanthamoeba polyphaga mimivirus

Acanthamoeba polyphaga mimivirus (APMV) is the only species in the Mimivirus genus of the Mimiviridae family of DNA viruses. It infects the amoeba, Acanthamoeba polyphaga. Its non-enveloped icosahedral capsid is 400 nm in diameter, with protein filaments of 100 nm projecting from its surface. The APMV genome is a linear, double-stranded DNA molecule of around 1.1 megabases, encoding around 979 genes. This is comparable to the genome of some small bacteria. It encodes several proteins that had not been previously discovered in viruses, including aminoacyl tRNA synthetases.

APMV is as large as some small species of bacteria. When it was first discovered in 1992, it was thought to be a bacterium, and named Bradfordcoccus. APMV was not shown to be a virus until 2003, when it was the largest virus then discovered. It has since been overtaken by Megavirus chilensis, Pandoravirus and Pithovirus, all of which also infect amoebae. These and other large viruses have been called "nucleocytoplasmic large DNA viruses", an informal grouping of very large and complex DNA viruses.

Did you know?

A giant virus factory infected with Zamilon

Selected biography

George Keble Hirst (2 March 1909 – 22 January 1994) was an American virologist who was among the first to study the molecular biology and genetics of animal viruses.

Hirst started to work on influenza virus in 1940, only a few years after it had been isolated. He soon discovered that the virus caused red blood cells to clump together. This phenomenon could be used to diagnose influenza, which had previously required growing the virus in ferrets. He invented the haemagglutination assay, a simple method for quantifying viruses, and later the haemagglutination inhibition assay, which measures virus-specific antibodies in serum. In 1942, he discovered the neuraminadase enzyme, showing for the first time that viruses could contain enzymes. Neuraminidase is the target of the neuraminidase inhibitor class of antiviral drugs, including oseltamivir and zanamivir. In 1962, he was also the first to propose the then-revolutionary idea that virus genomes can consist of discontinuous segments.

He co-founded Virology in 1955, the first English-language journal to focus on viruses, and directed the Public Health Research Institute in New York City for nearly 25 years (1956–81).

In this month

Louis Pasteur in 1878

1 July 1796: Edward Jenner first challenged James Phipps with variolation, showing that cowpox inoculation is protective against smallpox

3 July 1980: Structure of southern bean mosaic virus solved by Michael Rossmann and colleagues

6 July 1885: Louis Pasteur (pictured) gave rabies vaccine to Joseph Meister

10 July 1797: Jenner submitted paper on Phipps and other cases to the Royal Society; it was read to the society but not published

14–20 July 1968: First International Congress for Virology held in Helsinki

16 July 2012: FDA approved tenofovir/emtricitabine (Truvada) for prophylactic use against HIV; first prophylactic antiretroviral

19 July 2013: Pandoravirus described, with a genome twice as large as Megavirus

24–30 July 1966: International Committee on Nomenclature of Viruses (later the ICTV) founded

25 July 1985: Film star Rock Hudson made his AIDS diagnosis public, increasing public awareness of the disease

28 July 2010: First global World Hepatitis Day

Selected intervention

Child receiving the oral polio vaccine

Two polio vaccines are used against the paralytic disease polio. Each vaccine has benefits and disadvantages. The first, developed by Jonas Salk, consists of inactivated poliovirus. Based on three wild virulent strains, inactivated using formalin, it is administered by injection. 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, consists of three live virus strains, attenuated by growth in cell culture. They contain multiple mutations, stopping them from replicating in the nervous system. The Sabin vaccine provides longer-lasting immunity than the Salk vaccine, and can be administered orally, making it more suitable for mass vaccination campaigns. In around 1 in 750,000 people, 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 22 in 2017, and eradicated the disease from most countries.



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