Portal:Viruses

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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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Selected disease

Cold sore on the lower lip (arrow)

Herpes simplex is caused by herpes simplex virus types 1 and 2 of the Herpesviridae family. The most common forms of infection are oral herpes, which results in cold sores, and genital herpes. Active disease usually involves blisters containing infectious virus, although the genital form is often asymptomatic. Rare disorders associated with herpes simplex include herpetic whitlow, herpes gladiatorum, ocular herpes, cerebral herpes infection encephalitis, Mollaret's meningitis, neonatal herpes and possibly Bell's palsy.

After initial infection, virus particles are transported along sensory nerves to the sensory nerve cell bodies, where they become latent and remain lifelong. Periods of remission alternate with outbreaks of active disease lasting 2–21 days, in which the virus multiplies in the nerve cell and new virus particles are transported along the neuronal axon to the nerve terminals in the skin, where they are released. What causes these recurrences is unclear, although immunosuppressants are known to be one trigger. Transmission is usually by direct contact with a lesion or with body fluids, and can occur during periods of asymptomatic shedding. Barrier protection methods reduce the risk. No cure exists, but antiviral treatment can alleviate symptoms and reduce viral shedding.

Selected picture

Aedes aegypti mosquito biting a human

Aedes aegypti can transmit the chikungunya, dengue and yellow fever viruses. Mosquito control is key to disease prevention.

Credit: United States Department of Agriculture (2000)

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. 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

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 polio cases from around 350,000 in 1988 to just 223 in 2012, and eradicated the disease from most countries.

In this month

Plaques of lambda phage growing on Escherichia coli

1 December 1988: First World AIDS Day

4 December 1915: Frederick Twort discovered bacteriophages

6 December 1995: Saquinavir approved by FDA; the first HIV protease inhibitor

6 December 2013: Sofosbuvir approved for treatment of chronic hepatitis C virus (HCV), the first HCV nucleotide analogue and the first drug approved for interferon-free treatment

9 December 1979: Global Commission for Certification of Smallpox Eradication signed document formally certifying smallpox eradication

15 December 1955: Crystallisation of poliovirus by Fred Schaffer and Carlton Schwerdt, the first animal virus to be crystallised

15 December 1967: Infectious phi X 174 synthesised by Arthur Kornberg and coworkers, the first synthetic virus

18 December 1908: Poliovirus discovered by Karl Landsteiner and Erwin Popper

25 December 1982: Lambda phage (plaques pictured) sequenced by Fred Sanger and coworkers

29 December 1926: Thomas Milton Rivers proposed that viruses are obligate parasites

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Selected virus

X-ray crystallographic structure of the Norwalk virus capsid

Noroviruses are a genus of non-enveloped, single-stranded RNA viruses in the Caliciviridae family. The positive-sense RNA genome is approximately 7.5 kbp. Known noroviruses fall into five different genogroups (GI–GV); three groups infect humans, the other two mice, cattle and other bovines. All are considered strains of a single species, Norwalk virus.

Noroviruses are extremely contagious, with fewer than 20 virus particles being infectious. They are transmitted directly from person to person and indirectly via contaminated water and food. After infection, the virus replicates in the small intestine, causing acute gastroenteritis, which develops 24–48 hours after exposure and lasts for 24–60 hours. The characteristic symptoms include nausea, forceful vomiting, watery diarrhoea and abdominal pain. Infection is usually self-limiting and rarely severe. Noroviruses are the most common cause of viral gastroenteritis in humans, affecting around 267 million people a year and causing over 200,000 deaths, mainly in less-developed countries and in very young, elderly or immunosuppressed people. Hand washing with soap and water is effective in reducing transmission.

Selected article

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.

Did you know?

Jandokot Memorial to the "Sugarbird Lady", Robin Miller

Selected biography

Sir Frank Macfarlane Burnet (1945)

Sir Frank Macfarlane Burnet (3 September 1899 – 31 August 1985) was an Australian virologist, microbiologist and immunologist. His early virological studies were on bacteriophages, including the pioneering observation that bacteriophages could exist as a stable non-infectious form that multiplies with the bacterial host, later termed the lysogenic cycle.

With the outbreak of World War II, Burnet's focus moved to influenza. Although his efforts to develop a live vaccine proved unsuccessful, he developed assays for the isolation, culture and detection of influenza virus, including hemagglutination assays. Modern methods for producing influenza vaccines are still based on his work improving virus-growing processes in hen's eggs. He also researched influenza virus genetics, examining the genetic control of virulence and demonstrating, several years before influenza virus was shown to have a segmented genome, that the virus recombined at high frequency.

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