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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

Diagram showing the three layers of the meninges: the dura mater (blue), arachnoid mater (green) and pia mater (fawn)

Meningitis is an inflammation of the meninges, protective membranes covering the brain and spinal cord. The most common cause is infection with viruses including enteroviruses, herpes simplex virus (mainly type 2), varicella zoster virus, mumps virus, HIV and lymphocytic choriomeningitis. Meningitis can also be caused by infection with bacteria, fungi and protozoa, as well as several non-infectious causes. The most common symptoms are headache and neck stiffness associated with fever, confusion or altered consciousness, vomiting, and an inability to tolerate light (photophobia) or loud noises (phonophobia). Children often have only nonspecific symptoms, such as irritability and drowsiness. Meningitis is potentially life-threatening, and can lead to serious long-term consequences such as deafness, epilepsy, hydrocephalus and cognitive deficits. Viral meningitis is generally more benign than that caused by bacterial infection; it usually resolves spontaneously and is rarely fatal.

Polymerase chain reaction (PCR) of cerebrospinal fluid and identification of antibodies can be used to differentiate between viral causes. Viral meningitis typically only requires supportive therapy; meningitis caused by herpes simplex virus or varicella zoster virus sometimes responds to treatment with antiviral drugs such as aciclovir. Mumps-associated meningitis can be prevented by vaccination.

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Lady Montagu in Turkish dress by Jean-Étienne Liotard (c. 1756)

Lady Mary Wortley Montagu survived smallpox, and popularised the Turkish practice of variolation against the disease in western Europe in the 1720s.

Credit: Jean-Étienne Liotard (c. 1756)

Selected article

Diagram showing adaptive immunity and memory

The immune system is a system of structures and processes within an organism that protects against disease. It must detect a wide variety of pathogens – from viruses to parasitic worms – distinguish them from the organism's own healthy tissue, and neutralise them. Simple unicellular organisms such as bacteria have enzymes that protect against bacteriophage infections. Other basic immune mechanisms, including phagocytosis and the complement system, evolved in ancient eukaryotes.

Humans and most other vertebrates have more sophisticated defence mechanisms, including the ability to adapt over time to recognise specific pathogens more efficiently. Adaptive immunity creates immunological memory after an initial response to a specific pathogen, leading to an enhanced response to subsequent encounters with that same pathogen. This process of acquired immunity is the basis of vaccination. Viruses and other pathogens can rapidly evolve to evade immune detection, and some viruses, notably HIV, cause the immune system to function less effectively.

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

Quarantine notices at the East Birmingham Hospital where the first case was initially treated

The last recorded smallpox death occurred during the 1978 smallpox outbreak in Birmingham, UK. The outbreak resulted from accidental exposure to Variola major, probably the Abid strain, from a WHO-funded laboratory, headed by Henry Bedson, at the University of Birmingham Medical School – also the probable source of a 1966 outbreak. Bedson was investigating strains of smallpox known as whitepox, considered a potential threat to the smallpox eradication campaign, then in its final stages. The virus appears to have spread between floors in late July, possibly via ducting, to infect a medical photographer who worked above the laboratory. She showed symptoms in August and died the following month; one of her contacts was also infected but survived. Bedson committed suicide while under quarantine.

A government inquiry into the outbreak criticised the university's safety procedures. Radical changes in UK research practices for handling dangerous pathogens followed, and all known stocks of smallpox virus were concentrated in two laboratories.

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Selected virus

Electron micrograph of cauliflower mosaic virus particles

Cauliflower mosaic virus (CaMV) is a plant pararetrovirus in the Caulimoviridae family, which has similarities with hepadnaviruses such as hepatitis B virus. It predominantly infects members of the Brassicaceae (cabbage) family, including cauliflower and turnip; some strains can also infect Datura and Nicotiana species of the Solanaceae family. It is transmitted by aphid vectors, such as Myzus persicae. Symptoms include a mottled leaf pattern called "mosaic", necrotic lesions on the surface of infected leaves, stunted growth and deformation of the overall plant structure.

Although the viral genome is double-stranded DNA, the virus replicates via reverse transcription like a retrovirus. The icosahedral virion is 52 nm in diameter, and is built from 420 capsid protein subunits. The circular 8 kb genome encodes seven proteins, including a movement protein, which facilitates viral movement to neighbouring cells, and an insect transmission factor, which recognises a protein receptor at the tip of the aphid mouthparts. CaMV has several ways of evading the host defensive responses, which include interrupting salicylic acid-dependent signalling and decoying host silencing machinery. The virus has a strong constitutive (always on) promoter, CaMV 35S, which is widely used in plant genetic engineering.

Did you know?

Clara Maass

Selected biography

Randy Shilts (8 August 1951 – 17 February 1994) was an American journalist, author and AIDS activist. The first openly gay reporter for a mainstream US newspaper, Shilts covered the unfolding story of AIDS and its medical, social, and political ramifications from the first reports of the disease in 1981. New York University's journalism department later ranked his 1981–85 AIDS reporting in the top fifty works of American journalism of the 20th century. His extensively researched account of the early days of the epidemic in the US, And the Band Played On: Politics, People, and the AIDS Epidemic, first published in 1987, brought him national fame. The book won the Stonewall Book Award and was made into an award-winning film. Shilts saw himself as a literary journalist in the tradition of Truman Capote and Norman Mailer. His writing has a powerful narrative drive, and interweaves personal stories with political and social reporting.

He received the 1988 Outstanding Author award from the American Society of Journalists and Authors, the 1990 Mather Lectureship at Harvard University, and the 1993 Lifetime Achievement Award from the National Lesbian and Gay Journalists' Association. He died of AIDS in 1994.

In this month

Diagram of the human rhinovirus capsid

1 September 1910: Peyton Rous shows that a sarcoma of chickens, subsequently associated with Rous sarcoma virus, is transmissible

3 September 1917: Discovery of bacteriophage of Shigella by Félix d'Herelle

8 September 1976: Death of Mabalo Lokela, the first known case of Ebola virus

8 September 2015: Discovery of giant virus Mollivirus sibericum in Siberian permafrost

11 September 1978: Janet Parker was the last person to die of smallpox

12 September 1957: Interferon discovered by Alick Isaacs and Jean Lindenmann

12 September 1985: Structure of human rhinovirus 14 (pictured) solved by Michael Rossmann and colleagues, the first atomic-level structure of an animal virus

17 September 1999: Jesse Gelsinger died in a clinical trial of gene therapy using an adenovirus vector, the first known death due to gene therapy

20 September 2015: Wild poliovirus type 2 declared eradicated

26 September 1997: Combivir (zidovudine/lamivudine) approved; first combination antiretroviral

27 September 1985: Structure of poliovirus solved by Jim Hogle and colleagues

28 September 2007: First World Rabies Day is held

Selected intervention

The MMR vaccine controversy centered around the – now discredited – notion that the combined vaccine for measles, mumps and rubella (MMR) might be associated with colitis and autism spectrum disorders. The idea was based on a research paper by Andrew Wakefield and co-authors, published in the prestigious medical journal The Lancet in 1998, and subsequently shown to be fraudulent. Sunday Times journalist Brian Deer's investigations revealed that Wakefield had manipulated evidence and had multiple undeclared conflicts of interest. The Lancet paper was retracted in 2010; Wakefield was found guilty of serious professional misconduct by the General Medical Council, and struck off the UK's Medical Register. The claims in Wakefield's article were widely reported in the press, resulting in a sharp drop in vaccination uptake in the UK and Ireland. A significantly increased incidence of measles and mumps followed, leading to deaths and severe injuries. Multiple large epidemiological studies have found no link between the vaccine and autism.

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