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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".

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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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A man sneezing

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

Credit: James Gathany (2009)

In the news

Electron micrograph of Zaire ebolavirus

8 August: WHO declares the ongoing West African outbreak of Ebola virus disease (virus pictured) to be an international public health emergency; since the outbreak began, there have been more than 1750 suspected cases and 961 deaths. WHO

20 July: A study in rhesus monkeys suggests that after rectal infection with simian immunodeficiency virus, a model for HIV infection, the virus reaches hard-to-treat reservoirs before it can be detected in the blood, posing a challenge for eradication. Nature

17 July: At least six HIV/AIDS researchers and activists travelling to the World AIDS Conference in Melbourne are killed when Malaysia Airlines Flight 17 crashes; confirmed casualties are Joep Lange (University of Amsterdam/Amsterdam Institute for Global Health and Development), Jacqueline van Tongeren (Amsterdam Institute for Global Health and Development), Pim de Kuijer and Martine de Schutter (Aids Fonds/STOP AIDS NOW!), Lucie van Mens (The Female Health Company) and Glenn Thomas (World Health Organisation). IAS

Electron micrograph of Lassa virus particles

11 July: Three injections of a tetravalent dengue vaccine confer better protection than placebo in a phase III trial among over 10,000 children in endemic areas of Asia. Lancet

6 July: Hundreds of novel bacteriophage species are discovered in human gut flora samples. Nat Biotechnol

27 June: Entry of Lassa virus (pictured) into susceptible cells is shown to be a two-step process, with the virus binding to a second receptor, LAMP1, located inside lysosomes. Science

25 June: The human monoclonal antibody m102.4 is an effective treatment for Nipah virus in an African green monkey model. Sci Transl Med

Selected intervention

Ball-and-stick model of zidovudine

Zidovudine (ZDV) (also known as AZT and Retrovir) is an antiretroviral drug used in the treatment of HIV/AIDS. Classed as a nucleoside analogue reverse-transcriptase inhibitor, it inhibits HIV's reverse transcriptase enzyme, which copies the viral RNA into DNA and is essential for its replication. The first breakthrough in AIDS therapy, ZDV was licensed in 1987. While it significantly reduces HIV replication, leading to clinical and immunological benefits, when used alone ZDV does not completely stop replication, allowing the virus to become resistant to it. The drug is therefore used together with other anti-HIV drugs in combination therapy called highly active antiretroviral therapy (HAART). To simplify its administration, ZDV is included in Combivir, Trizivir and other combination pills. ZDV can also be used to prevent HIV transmission, such as from mother to child during childbirth or after a needlestick injury.

In this month

Electron micrograph of Ebola virus

6 August 2007: Maraviroc, first CCR5 receptor antagonist, approved for HIV/AIDS

8 August 2011: UN declared rinderpest eradicated

18 August 1990: Ryan White Care Act enacted, the largest American federally funded programme for people living with HIV/AIDS

26 August 1976: First case of Ebola virus (pictured), now the Zaire form

26 August 1998: Fomivirsen, first antisense drug, approved for cytomegalovirus retinitis

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X-ray crystallographic structure of the bovine papillomavirus capsid

Papillomaviruses are non-enveloped DNA viruses that make up the Papillomaviridae family. Their circular double-stranded genome is around 8 kb. They infect humans, other mammals and some other vertebrates. All papillomaviruses replicate exclusively in epithelial cells of stratified squamous epithelium, which forms the skin and some mucosal surfaces, including the mouth, airways, genitals and anus.

Infection by most papillomaviruses is either asymptomatic or causes small benign tumours known as warts or papillomas. Francis Peyton Rous showed in 1935 that a papillomavirus could cause skin cancer in rabbits – the first time that a virus was shown to cause cancer in mammals – and papillomas caused by some virus types, including human papillomavirus (HPV) 16 and 18, carry a risk of becoming cancerous if the infection persists. Papillomaviruses are associated with cancers of the cervix, vulva, vagina, penis, oropharynx and anus in humans. Cervical cancer can be detected by screening, but has substantial mortality in developing countries; worldwide, an estimated 490,000 cases and 270,000 deaths occur each year. HPV vaccines protect against types 16 and 18, which cause the majority of cancers.

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T4 bacteriophage, typical of myovirus bacteriophages

Bacteriophages (or phages) are a large and diverse group of viruses that infect bacteria and archaea. Their genome, which they inject into the host's cytoplasm, can be DNA or RNA, single or double stranded, linear or circular, and contains between four and several hundred genes. Their capsid can be relatively simple or elaborate in structure, and in a few groups is surrounded by an envelope. Caudovirales, double-stranded DNA phages with tails, is the best-studied group, and includes T4 (pictured) and λ phage.

Among the most common entities in the biosphere, bacteriophages are widely distributed in locations populated by bacteria, such as soil and animal intestines. One of the densest natural sources is sea water, where up to 900 million virions/mL have been found in microbial mats at the surface, and up to 70% of marine bacteria can be infected.

Used as an alternative to antibiotics for over 90 years, phages might offer a potential therapy against multi-drug-resistant bacteria.

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Baby Asian elephant

Selected biography

Edward Jenner, by James Northcote (1800s)

Edward Jenner (17 May 1749 – 26 January 1823) was an English physician and scientist from Berkeley, Gloucestershire, who was the pioneer of the smallpox 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. Jenner tested his hypothesis by inoculating an eight-year-old boy with such pus. He subsequently repeatedly challenged the boy with variolous material, then the standard method of immunisation, without inducing disease. Although 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 the work of any other man".

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