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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

Light microscope image of an H&E-stained liver biopsy, showing "ground glass hepatocytes" associated with chronic hepatitis B infection

Hepatitis B is an infectious inflammatory disease of the liver caused by the hepatitis B virus (HBV), a hepadnavirus. It affects humans and possibly other great apes. The virus is transmitted by exposure to infectious blood or some body fluids. Mother-to-child transmission is a major route in endemic countries. HBV is 50–100 times more infectious than HIV. The virus replicates in liver cells, and enters the blood where viral proteins and antiviral antibodies are found.

Acute infection is often asymptomatic but can cause liver inflammation resulting in vomiting, jaundice and, rarely, death. Over 95% of infected adults and older children clear the infection spontaneously, developing protective immunity. Only 30% of children aged 1–6 years and 5% of newborns infected perinatally clear the infection. Chronic hepatitis B may eventually progress to cirrhosis and liver cancer, causing death in around 40% of those chronically infected. The virus has infected humans since at least the Bronze Age, with HBV DNA being found in 4,500-year-old human remains. About a third of the global population has been infected at one point in their lives, including nearly 350 million who are chronic carriers. The virus is endemic in East Asia and sub-Saharan Africa. Infection can be prevented by vaccination.

Selected image

A man sneezing

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

Credit: James Gathany (2009)

Selected article

Tobacco mosaic virus on a tobacco leaf, showing the characteristic mottling

Plant viruses face different challenges from animal viruses. As plants do not move, transmission between hosts often involves vectors, most commonly insects, but also nematodes and protozoa. Plant viruses can also spread via seeds, and by direct transfer of sap. Plant cells are surrounded by cell walls which are difficult to penetrate. Movement between cells occurs mainly by transport through plasmodesmata, and most plant viruses encode movement proteins to make this possible. Although plants lack an adaptive immune system, they have complex defences against viral infection. Viruses of cultivated plants often cause disease, and are thought to cause up to US$60 billion losses to global crop yields each year; infections of wild plants are often asymptomatic.

Most plant viruses are rod-shaped, with protein discs forming a tube surrounding the viral genome; isometric particles are another common structure. They rarely have an envelope. The great majority have an RNA genome, which is usually small and single stranded. Plant viruses are grouped into 73 genera and 49 families. Tobacco mosaic virus (pictured) is among the best characterised of the 977 species officially recognised in 1999.

In the news

Diagram of African swine fever virus

2 July: In the ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo (DRC) – now the second-largest Ebola outbreak in history – new cases remain stable, with 285 reported in the past 3 weeks, and a total of 2372 cases, including 1602 deaths, since the outbreak began in August 2018. WHO 1, 2

26 June: A meta-analysis of vaccinating girls and women against human papillomavirus including 60 million vaccinees finds that diagnoses of high-grade cervical intraepithelial neoplasia and cervical cancer in women and of anogenital warts in both women and men have reduced in frequency. Lancet

14 June: The WHO Emergency Committee declares that the ongoing DRC Ebola virus outbreak fails to meet the criteria for a Public Health Emergency of International Concern, despite the spread to Uganda. WHO

3 May: In the ongoing Rift Valley fever outbreak in the Mayotte Islands in the Comoro group there have been 129 confirmed cases since the outbreak started in November 2018. WHO

2 May: A European observational study in 972 gay male couples finds no HIV transmission with unprotected sex where the HIV-positive partner's virus is fully suppressed by antiretroviral therapy. Lancet

25 April: A major outbreak of African swine fever ongoing in pigs in China since August 2018 has caused the loss of at least 40 million pigs, and the virus (pictured) has also been reported elsewhere in Southeast Asia. BBC

15 April: The directors of WHO and UNICEF warn that the more than 110,000 measles cases reported globally in January–March represent a nearly threefold increase over the same period in 2018. CNN

14 April: In the ongoing chikungunya virus outbreak in Congo, 6,149 suspected cases have been reported since the outbreak began in January, with nearly half in Kouilou Department. WHO

29 March: The filamentous bacteriophage Pf is shown to increase the pathogenicity of its bacterial host, Pseudomonas aeruginosa, an important human pathogen, by protecting it from the immune system in a mouse model. Science

12 March: The plant nanovirus, faba bean necrotic stunt virus – which has a segmented (multi-part) genome, with each of the eight segments being packaged separately – is shown to be able to replicate successfully even when its DNA segments do not all enter the same cell. eLife

8 March: The International Committee on Taxonomy of Viruses ratifies an update to virus classification, creating the Riboviria taxon for all RNA viruses at the new rank of realm. ICTV

5 March: Another case of apparent clearance of HIV from an infected patient after stem-cell therapy is reported. Nature

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).

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

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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?

HMAT Boonah

Selected biography

Peter Piot in 2006

Peter Piot (born 17 February 1949) is a Belgian virologist and public health specialist, known for his work on Ebola virus and HIV.

During the first outbreak of Ebola in Yambuku, Zaire in 1976, Piot was one of a team that discovered the filovirus in a blood sample. He and his colleagues travelled to Zaire to help to control the outbreak, and showed that the virus is transmitted via blood and during preparation of bodies for burial. He advised WHO during the West African Ebola epidemic of 2014–16.

In the 1980s, Piot participated in collaborative projects in Burundi, Côte d'Ivoire, Kenya, Tanzania and Zaire, including Project SIDA in Kinshasa, the first international project on AIDS in Africa, which provided the foundations for understanding HIV infection in that continent. He was the founding director of UNAIDS, and has served as president of the International AIDS Society and assistant director of the WHO Global HIV/AIDS Programme. As of 2019, he directs the London School of Hygiene & Tropical Medicine.

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 and autism fraud refers to the false claim that the combined vaccine for measles, mumps and rubella (MMR) might be associated with colitis and autism spectrum disorders. Multiple large epidemiological studies have since found no link between the vaccine and autism. The notion originated in a fraudulent research paper by Andrew Wakefield and co-authors, published in the prestigious medical journal The Lancet in 1998. Sunday Times journalist Brian Deer's investigations revealed that Wakefield had manipulated evidence and had multiple undeclared conflicts of interest. The paper was retracted in 2010, when the Lancet's editor-in-chief Richard Horton characterised it as "utterly false". 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 greatly increased incidence of measles and mumps followed, leading to deaths and serious permanent injuries.

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