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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

Child with a measles rash

Measles is a disease that only affects humans caused by the measles virus, an RNA virus in the Paramyxoviridae family. It is highly contagious, with transmission occurring via the respiratory route or by contact with secretions. Symptoms generally develop 10–12 days after exposure and last 7–10 days; they include high fever, cough, rhinitis and conjunctivitis, white Koplik's spots inside the mouth and a generalised red maculopapular rash. Complications occur in about 30% of those infected, and include diarrhoea, pneumonia, bronchitis, otitis media, encephalitis, corneal ulceration and blindness. The risk of death is usually around 0.1–0.3%, but may be as high as 10–28% in areas with high levels of malnutrition.

Measles was first described by Rhazes (860–932). The disease is estimated to have killed around 200 million people between 1855 and 2005. It affects about 20 million people a year, primarily in the developing areas of Africa and Asia; as of 2013, it causes the most vaccine-preventable deaths of any disease, at about 96,000 annually. No antiviral drug is licensed. An effective measles vaccine is available, but uptake has been reduced by anti-vaccination campaigns, particularly the fraudulent claim that the MMR vaccine might be associated with autism.

Selected picture

A man sneezing

Transmission via the respiratory route is important for many viruses, including influenza, measles and varicella zoster virus.

Credit: James Gathany (2009)

Selected article

Culex mosquitoes are the vectors for West Nile Virus

Infectious diseases are symptomatic diseases resulting from the infection and replication of pathogens, including viruses, prions, bacteria, fungi, protozoa and multicellular parasites, in an individual host. Infectious diseases were responsible for over a quarter of human deaths globally in 2002, with HIV, measles and influenza being among the most significant viral causes of death.

Infectious pathogens must enter, survive and multiply within the host, and spread to fresh hosts. Relatively few microorganisms cause disease in healthy individuals, and infectious disease results from the interplay between these rare pathogens and the host's defences. Infection does not usually result in the host's death, and the pathogen is generally cleared from the body by the host's immune system. Transmission can occur by physical contact, contaminated food, body fluids, objects, airborne inhalation or via vectors, such as the mosquito (pictured). Diagnosis often involves identifying the pathogen; techniques include culture, microscopy, immunoassays and PCR-based molecular diagnostics.

In the news

Cryo-electron micrograph of Zika virus

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

Sulphur-crested cockatoo with beak and feather disease

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

Transmission electron micrograph of ranaviruses infecting a cell

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

Selected outbreak

The masked palm civet (Paguma larvata) is thought to have been the source of SARS coronavirus

In the severe acute respiratory syndrome (SARS) outbreak, the first cases of the newly emerged SARS coronavirus were reported in November 2002 from the Chinese Guangdong province. The virus soon spread across Asia, with China, Hong Kong, Taiwan and Singapore being the worst affected countries; a secondary outbreak occurred in Canada. Over 8,000 people were infected, with nearly 10% dying. Those over 50 years had a much higher mortality rate, approaching half. The outbreak was contained by July 2003.

The immediate source of SARS coronavirus is likely to have been the masked palm civet (Paguma larvata), which was sold as food in Guangdong markets. The virus was also found in raccoon dogs, ferret badgers and domestic cats, and closely related coronaviruses have been isolated from bats, which probably form the natural reservoir. The rapid initial spread of the outbreak has been in part attributed to China's slow response to the early cases.

Selected quotation

Donald McNeil on the campaign to eradicate polio

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Selected virus

Diagram of hepatitis D virus

Hepatitis D virus or hepatitis delta virus (HDV) is a small RNA virus, the sole member of the Deltavirus genus. It infects humans. A satellite virus, it can only replicate in the presence of a hepatitis B (HBV) helper virus. The spherical virion is 36 nm in diameter, with an envelope containing three HBV proteins. The single-stranded, negative-sense, circular RNA genome of 1679 nucleotides is the smallest genome of any known animal virus. It has an unusual base composition for a virus that infects animals, and is extensively bound to itself to form a rod-shaped structure. These features have led to suggestions that HDV might be related to viroids, small unencapsidated circular RNAs that infect plants. Unlike viroids, HDV encodes a protein, hepatitis D antigen.

Both HDV and HBV enter liver cells using the sodium/bile acid cotransporter as their receptor. They are mainly transmitted via injecting drug use and blood products. An estimated 15–20 million people are infected with both viruses, which is associated with an increased risk of liver complications. Around one in five jointly infected patients die. The HBV vaccine protects against HDV.

Did you know?

Harald zur Hausen, photographed by Armin Kübelbeck

Selected biography

Aniru Conteh

Aniru Sahib Sahib Conteh (6 August 1942 – 4 April 2004) was a Sierra Leonean physician and expert on the clinical treatment of Lassa fever, a viral haemorrhagic fever endemic to West Africa.

He joined the Centers for Disease Control and Prevention Lassa fever programme in Segbwema, first as superintendent and then as clinical director. After the Sierra Leone Civil War began in 1991, Conteh moved to the Kenema Government Hospital, where he spent the next two decades running the only dedicated Lassa fever ward in the world. He collaborated with the British charity Merlin to promote public health in Sierra Leone through education and awareness campaigns intended to prevent Lassa fever. With little funding and few supplies, he successfully reduced mortality rates and saved many lives until an accidental needlestick injury led to his own death from the disease in 2004.

Conteh received renewed public attention in 2009 as the hero of Ross I. Donaldson's memoir, The Lassa Ward.

In this month

Smallpox vaccination kit, including the bifurcated needle used to administer the vaccine

1 January 1934: Discovery of mumps virus by Claud Johnson and Ernest Goodpasture

1 January 1942: Publication of George Hirst's paper on the haemagglutination assay

1 January 1967: Start of WHO intensified eradication campaign for smallpox (vaccination kit pictured)

3 January 1938: Foundation of March of Dimes, to raise money for polio

6 January 2011: Andrew Wakefield's paper linking the MMR vaccine with autism described as "fraudulent" by the BMJ

25 January 1988: Foundation of the International AIDS Society

29 January 1981: Influenza haemagglutinin structure published by Ian Wilson, John Skehel and Don Wiley, the first viral membrane protein whose structure was solved

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 74 in 2015, and eradicated the disease from most countries.

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