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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

Shingles rash on the chest

Shingles, or herpes zoster, is a painful skin rash with blisters that, characteristically, occurs in a stripe limited to just one side of the body. The rash usually heals within 2–5 weeks, but around one in five people experience residual nerve pain for months or years.

Shingles is caused by varicella zoster virus (VZV), an alpha-herpesvirus. Initial VZV infection usually occurs in childhood causing chickenpox. After this resolves, the virus is not eliminated from the body, but remains latent in the nerve cell bodies of the dorsal root or trigeminal ganglia, without causing symptoms. Years or decades later, shingles occurs when virions in a single ganglion reactivate, travel down nerve fibres and infect the skin around the nerve. The shingles rash is restricted to the area of skin supplied by a single spinal nerve, termed the dermatome. Exactly how VZV remains latent in the body, and subsequently reactivates, is unclear.

Around a third of the population will develop shingles. Repeated episodes are rare. In the United States, about half the cases occur in people aged 50 years or older. Vaccination at least halves the risk, and prompt treatment with aciclovir or related antiviral drugs can reduce the severity and duration of the rash.

Selected image

Gods slaandehand over Nederland, door pest-siekte onder het rund vee by Jacobus Eussen (1745)

Rinderpest was a Morbillivirus that caused catastrophic cattle plagues for centuries. It was declared the second virus to have been eradicated globally in 2011.

Credit: Jacobus Eussen (1745)

In the news

Map showing the distribution of coronavirus cases; black: highest incidence; dark red to pink: decreasing incidence; grey: no recorded cases
Map showing the distribution of coronavirus cases; black: highest incidence; dark red to pink: decreasing incidence; grey: no recorded cases

4 April: The ongoing pandemic of a novel coronavirus is accelerating rapidly; more than a million confirmed cases, including more than 57,000 deaths, have been documented globally since the outbreak began in December 2019. WHO 1, 2

27 March: An international, randomised, non-blinded, clinical trial organised by the World Health Organization of four potential treatments for COVID-19remdesivir; chloroquine or hydroxychloroquine; lopinavir/ritonavir; or lopinavir/ritonavir plus interferon-beta – is about to start enrolling patients. Science, WHO

16 March: A phase I clinical trial of a messenger RNA-based vaccine candidate for the novel coronavirus begins in Seattle. NIH

11 March: The World Health Organization describes the ongoing outbreak of respiratory disease caused by a novel coronavirus as a pandemic. WHO

10 March: A patient with apparent clearance of HIV after stem-cell therapy continues to have no viable virus detectable in blood or other reservoirs after 30 months without antiretroviral treatment. Lancet

9 March: No new cases have been recorded in three weeks in the ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo; as of 3 March there had been a total of 3444 cases, including 2264 deaths, since the outbreak began in August 2018. WHO 1, 2

False-coloured electron micrograph of novel coronavirus
False-coloured electron micrograph of novel coronavirus

12 February: The ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo remains a Public Health Emergency of International Concern, according to the World Health Organization. WHO 1

7 February: Chinese scientists announce that novel coronavirus (2019-nCoV) is 99% identical to a coronavirus isolated from pangolins, suggesting these animals might be an intermediate host. Nature

5 February: A study of 2658 samples from 38 different types of cancer found that 16% were associated with a virus, higher than previous estimates, but did not identify any new candidate tumour viruses. Nat Genet

4 February: Over 2500 putative circular DNA virus genomes are catalogued from metagenomic surveys of human and animal samples, including over 600 dissimilar to existing virus groups. eLife, Science

3 February: The US National Institute of Allergy and Infectious Diseases stops the South African HVTN 702 Phase IIb/III clinical trial of an investigational HIV vaccine early, after the vaccine failed to prevent HIV infection. NIH

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, antimicrobial peptides called defensins, and the complement system, evolved in ancient eukaryotes and are found in plants and invertebrates.

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.

Selected outbreak

The deer mouse was the reservoir for Sin Nombre hantavirus in the Four Corners outbreak.

The 1993 hantavirus outbreak in the Four Corners region of southwest USA was of a novel hantavirus, subsequently named Sin Nombre virus. It caused the previously unrecognised hantavirus pulmonary syndrome – the first time that a hantavirus had been associated with respiratory symptoms. Mild flu-like symptoms were followed by the sudden onset of pulmonary oedema, which was fatal in half of those affected. A total of 24 cases were reported in April–May 1993, with many of those affected being from the Navajo Nation territory. Hantavirus infection of humans generally occurs by inhaling aerosolised urine and faeces of rodents, in this case the deer mouse (Peromyscus; pictured).

Previously documented hantavirus disease had been confined to Asia and Europe, and these were the first human cases to be recognised in the USA. Subsequent investigation revealed undiagnosed cases dating back to 1959, and Navajo people recalled similar outbreaks in 1918, 1933 and 1934.

Selected quotation

Recommended articles

Viruses & Subviral agents: elephant endotheliotropic herpesvirus • HIV • introduction to virusesFeatured article • Playa de Oro virus • poliovirus • prion • rotavirusFeatured article • virusFeatured article

Diseases: colony collapse disorder • common cold • croup • dengue feverFeatured article • gastroenteritis • Guillain–Barré syndrome • hepatitis B • hepatitis C • hepatitis E • herpes simplex • HIV/AIDS • influenzaFeatured article • meningitisFeatured article • myxomatosis • poliomyelitisFeatured article • pneumonia • shingles • smallpox

Epidemiology & Interventions: 2007 Bernard Matthews H5N1 outbreak • 2009 flu pandemic • HIV/AIDS in Malawi • polio vaccine • Spanish flu • West African Ebola virus epidemic

Virus–Host interactions: antibody • host • immune systemFeatured article • parasitism • RNA interferenceFeatured article

Methodology: metagenomics

Social & Media: And the Band Played On • Contagion • "Flu Season" • Frank's CockFeatured article • Race Against Time: Searching for Hope in AIDS-Ravaged AfricaFeatured article • social history of virusesFeatured article • "Steve Burdick" • "The Time Is Now"

People: Brownie Mary • Macfarlane BurnetFeatured article • Aniru Conteh • people with hepatitis CFeatured article • HIV-positive peopleFeatured article • Bette Korber • Henrietta Lacks • Linda Laubenstein • Barbara McClintockFeatured article • poliomyelitis survivorsFeatured article • Joseph Sonnabend • Eli Todd • Ryan WhiteFeatured article

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?

Painting of a sepia-coloured bat with prominent white patches on the shoulders of the wings and in the middle of its belly

Selected biography

Françoise Barré-Sinoussi in 2008

Françoise Barré-Sinoussi (born 30 July 1947) is a French virologist, known for being one of the researchers who discovered HIV.

Barré-Sinoussi researched retroviruses in Luc Montagnier's group at the Institut Pasteur in Paris. In 1982, she and her co-workers started to analyse samples from people with a new disease, then referred to as "gay-related immune deficiency". They found a novel retrovirus in lymph node tissue, which they called "lymphadenopathy-associated virus". Their results were published simultaneously with those of Robert Gallo's group in the USA, who had independently discovered the virus under the name "human T-lymphotropic virus type III". The virus, renamed "human immunodeficiency virus", was later shown to cause AIDS. Barré-Sinoussi continued to research HIV until her retirement in 2015, studying how the virus is transmitted from mother to child, the immune response to HIV, and how a small proportion of infected individuals, termed "long-term nonprogressors", can limit HIV replication without treatment. In 2008, she was awarded the Nobel Prize, with Montagnier, for the discovery of HIV.

In this month

Diagram of the bacteriophage MS2 capsid

1 April 1911: Peyton Rous showed that a cell-free isolate could transmit sarcoma in chickens, an early demonstration of cancer caused by a virus

7 April 1931: First electron micrograph taken by Ernst Ruska and Max Knoll

8 April 1976: Bacteriophage MS2 (pictured) sequenced by Walter Fiers and coworkers, first viral genome to be completely sequenced

8 April 1990: Death from AIDS of Ryan White, haemophiliac teenager for whom the Ryan White Care Act is named

8 April 1992: Tennis player Arthur Ashe announced that he had been infected with HIV from blood transfusions

9 April 1982: Stanley Prusiner proposed proteinaceous prions as the cause of scrapie

12 April 1955: Success of trial of Jonas Salk's polio vaccine announced

12 April 2013: New order of double-stranded DNA bacteriophages, Ligamenvirales, announced

15 April 1957: André Lwoff proposes a concise definition of a virus

21 April 1989: Discovery of hepatitis C virus by Qui-Lim Choo and colleagues

28 April 1932: First yellow fever vaccine announced at an American Societies for Experimental Biology meeting by Wilbur Sawyer

29 April 2015: PAHO and WHO declared the Americas region free from rubella transmission

30 April 1937: Discovery of Theiler's murine encephalomyelitis virus, later a model for multiple sclerosis research

Selected intervention

Ball-and-stick model of zidovudine

Zidovudine (ZDV) (also known as AZT and sold as Retrovir) is an antiretroviral drug used in the prevention and 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 some 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. To simplify its administration, ZDV is included in combination pills with lamivudine (Combivir) and lamivudine plus abacavir (Trizivir). ZDV continues to be used to prevent HIV transmission from mother to child during childbirth; it was previously part of the standard post-exposure prophylaxis after needlestick injury.

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