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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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Smallpox rash

Smallpox is an infectious disease of humans caused by the Variola major and V. minor viruses. V. major causes a more serious disease with a mortality rate of 30–35%; V. minor is associated with milder symptoms and below 1% mortality. The virus is mainly transmitted by the respiratory route. Smallpox localises in small blood vessels of the skin and in the mouth and throat. In the skin, this results in a characteristic maculopapular rash, and later, raised fluid-filled blisters. Long-term complications of V. major infection include characteristic scars, commonly on the face, which occur in 65–85% of survivors. Blindness resulting from corneal ulceration and scarring, and limb deformities due to arthritis and osteomyelitis are less common complications, seen in about 2–5% of cases.

Smallpox probably emerged in human populations in about 10,000 BC; the mummified body of Egyptian pharaoh Ramses V shows evidence of smallpox rash. The disease was responsible for an estimated 300–500 million deaths during the 20th century. Smallpox vaccine, the earliest vaccine, was developed in the 18th century, and intensive vaccination campaigns led to smallpox being declared the first infectious disease to be eradicated globally in 1979.

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Cartoon depicting cowpox vaccination by James Gillray (1802)

1802 cartoon of Edward Jenner administering cowpox vaccine against smallpox, satirising contemporary fears about vaccination.

Credit: James Gillray (12 June 1802)

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American soldiers with influenza H1N1 at a hospital ward at Camp Funston

The 1918 flu pandemic was an unusually deadly influenza pandemic, the first of the two involving H1N1 influenza virus. The pandemic's geographic origin is unknown. Lasting from January 1918 until December 1920, it infected 500 million people across the entire globe, with a death toll of 50–100 million (3–5% of the world's population), making it one of the deadliest natural disasters of human history. It was also implicated in the outbreak of encephalitis lethargica in the 1920s.

Most influenza outbreaks disproportionately kill young, elderly or already weakened patients; in contrast the 1918 pandemic predominantly killed healthy young adults. Modern research suggests that the virus kills through a cytokine storm, an overreaction of the body's immune system. The strong immune reactions of young adults resulted in a more severe disease with a higher mortality rate, whereas the weaker immune systems of children and older adults resulted in fewer deaths.

In the news

Electron micrograph of Zaire ebolavirus

25 February: WHO calls for increased measles vaccination coverage in the light of outbreaks across Europe; the ongoing outbreak in North America continues. WHO,CDC

24 February: A clinical trial in England shows that pre-exposure prophylaxis with tenofovir/emtricitabine (Truvada) reduces the risk of HIV infection by 86% in gay men engaging in high-risk sex. CROI

20 February: WHO approves the ReEBOV Antigen Rapid Test Kit, a protein-based diagnostic test for Ebola virus (pictured) that can give results in 15 minutes and does not require electricity. WHO

19 February: In the ongoing outbreak of Middle East respiratory syndrome coronavirus (MERS-CoV; pictured), an increase in weekly cases is reported in Saudi Arabia; since the outbreak started in September 2012, there have been over 1000 cases with at least 376 deaths. WHO

18 February: In a preliminary study, gene therapy with eCD4-Ig – an engineered protein that mimics two receptors for HIV, CD4 (pictured) and CCR5 – protects rhesus macaques against SHIV, a model for HIV. Nature

17 February: Four polio vaccine workers are found shot dead after being abducted in Pakistan. BBC

Graphic of Middle East respiratory syndrome coronavirus

16 February: A novel strain of HIV-1, CRF19_cpx, is associated with rapid progression to AIDS among some patients in Cuba. EBioMedicine

15 February: In the ongoing West African outbreak of Ebola virus disease, 128 new cases were confirmed, with transmission particularly high in Freetown, Sierra Leone; since the outbreak began, there have been over 23,000 suspected cases and 9380 deaths. WHO

14 February: An international collaboration to create a synthetic vaccine for poliovirus is announced. BBC

10 February: A novel RNA virus, Dinocampus coccinellae paralysis virus, is implicated in the parasitic wasp Dinocampus coccinellae paralysing its host, the spotted lady beetle, to form a bodyguard for its pupa. Proc. R. Soc. B

10 February: An antisense molecule targeting Ebolavirus VP24 mRNA protected 6 of 8 Rhesus monkeys from death from Ebola virus disease, suggesting that VP24 might be a target for anti-Ebola drugs. mBio

Ribbon diagram of human CD4

4 February: H7N9 avian influenza infection continues in China, with 83 cases reported. WHO

2 February: A Phase 2/3 clinical trial of two Ebolavirus vaccine candidates, cAd3-EBOZ and VSV-ZEBOV, commences in Liberia. NIH

30 January: A meta-analysis of all randomised clinical trials of oseltamivir (Tamiflu) in adults, including all published and unpublished data, shows that the antiviral reduces the duration of influenza symptoms by around one day in adults, and also reduces the risk of lower respiratory tract complications and the need for hospitalisation. Lancet

10 December: Gardasil 9, a nine-valent human papillomavirus (HPV) vaccine based on Gardasil that protects against an additional five high-risk HPV types, is approved by the FDA. FDA

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Electron micrograph of canine parvovirus

Canine parvovirus type 2 is a non-enveloped single-stranded DNA virus in the Parvoviridae family. The icosahedral viral capsid is only 20–26 nm in diameter, making it one of the smallest viruses. The genome is about 5000 nucleotides long. The virus is very similar to feline panleukopenia virus, another parvovirus, as well as mink enteritis and raccoon and fox parvoviruses. It infects dogs, wolves, foxes and other canids, big cats and occasionally domestic cats, but cannot infect humans.

A relatively new disease, canine parvovirus 2 infection was first recognised in 1978 and rapidly spread worldwide. The virus is transmitted by direct or indirect contact with faeces. The virus replicates in the lymphoid tissue in the throat, then spreads to the bloodstream to infect cells of the lymph nodes, intestinal crypts and bone marrow. The disease has two different types: the more common intestinal form causes severe vomiting and dysentery; the cardiac form affects in puppies under 8 weeks, causing respiratory or cardiovascular failure. Mortality can reach 91% in untreated cases. An effective vaccine is available.

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Ball-and-stick model of oseltamivir

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 haemagglutination 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.

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

Dmitri Ivanovsky

February 1939: First virology journal, Archiv für die gesamte Virusforschung, appeared

8 February 1951: Establishment of the HeLa cell line from a cervical carcinoma biopsy, the first immortal human cell line

12 February 1892: Dmitri Ivanovsky (pictured) demonstrated transmission of tobacco mosaic disease by extracts filtered through Chamberland filters; sometimes considered the beginning of virology

19 February 1966: Prion disease kuru shown to be transmissible

27 February 2005: H1N1 influenza strain resistant to oseltamivir reported in a human patient

24 February 1977: Phi X 174 sequenced by Fred Sanger and coworkers, the first virus and the first DNA genome to be sequenced

28 February 1998: Publication of Andrew Wakefield's Lancet paper, subsequently discredited, linking the MMR vaccine with autism, which started the MMR vaccine controversy



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