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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 6,000 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

Child with a measles rash

Measles is a disease that only affects humans caused by the measles virus, an RNA virus in the Paramyxoviridae family. It is highly contagious, with transmission occurring via the respiratory route or by contact with secretions. Symptoms generally develop 10–12 days after exposure and last 7–10 days; they include high fever, cough, rhinitis and conjunctivitis, white Koplik's spots inside the mouth and a generalised red maculopapular rash. Complications including diarrhoea, otitis media and pneumonia are relatively common; more rarely seizures, encephalitis, croup, corneal ulceration and blindness can occur. The risk of death is usually around 0.2%, but may be as high as 10–28% in areas with high levels of malnutrition and poor healthcare.

Measles was first described by Rhazes (860–932). The disease is estimated to have killed around 200 million people between 1855 and 2005. It affects about 20 million people a year, primarily in the developing areas of Africa and Asia, and is one of the leading vaccine-preventable disease causes of death. No antiviral drug is licensed. An effective measles vaccine is available, but uptake has been reduced by anti-vaccination campaigns, particularly the fraudulent claim that the MMR vaccine might be associated with autism. Rates of disease and deaths increased from 2017 to 2019, attributed to a decrease in immunisation.

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Egyptian stele believed to show a poliomyelitis survivor

This 18th Dynasty Egyptian stele, believed to show a priest with poliomyelitis-associated deformity, is one of the earliest records of a viral disease.

Credit: Unknown (1580–1350 BC)

In the news

Map showing the prevalence of SARS-CoV-2 cases; black: highest prevalence; dark red to pink: decreasing prevalence; grey: no recorded cases or no data
Map showing the prevalence of SARS-CoV-2 cases; black: highest prevalence; dark red to pink: decreasing prevalence; grey: no recorded cases or no data

26 February: In the ongoing pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), more than 110 million confirmed cases, including 2.5 million deaths, have been documented globally since the outbreak began in December 2019. WHO

18 February: Seven asymptomatic cases of avian influenza A subtype H5N8, the first documented H5N8 cases in humans, are reported in Astrakhan Oblast, Russia, after more than 100,0000 hens died on a poultry farm in December. WHO

14 February: Seven cases of Ebola virus disease are reported in Gouécké, south-east Guinea. WHO

7 February: A case of Ebola virus disease is detected in North Kivu Province of the Democratic Republic of the Congo. WHO

4 February: An outbreak of Rift Valley fever is ongoing in Kenya, with 32 human cases, including 11 deaths, since the outbreak started in November. WHO

21 November: The US Food and Drug Administration (FDA) gives emergency-use authorisation to casirivimab/imdevimab, a combination monoclonal antibody (mAb) therapy for non-hospitalised people twelve years and over with mild-to-moderate COVID-19, after granting emergency-use authorisation to the single mAb bamlanivimab earlier in the month. FDA 1, 2

18 November: The outbreak of Ebola virus disease in Équateur Province, Democratic Republic of the Congo, which started in June, has been declared over; a total of 130 cases were recorded, with 55 deaths. UN

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), λ phage and Mu phage.

Among the most common entities in the biosphere, bacteriophages are ubiquitous in locations populated by bacteria. 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.

Selected outbreak

Villagers in Yambuku, Zaire, being examined by staff from the US CDC

The 1976 Zaire Ebola virus outbreak was one of the first two recorded outbreaks of the disease. The causative agent was identified as a novel virus, named for the region's Ebola River. The first identified case, in August, worked in the school in Yambuku, a small rural village in Mongala District, north Zaire. He had been treated for suspected malaria at the Yambuku Mission Hospital, which is now thought to have spread the virus by giving vitamin injections with inadequately sterilised needles, particularly to women attending prenatal clinics. Unsafe burial practices also spread the virus.

The outbreak was contained by quarantining local villages, sterilising medical equipment and providing protective clothing to medical personnel, and was over by early November. A total of 318 cases was recorded, of whom 280 died, an 88% case fatality rate. An earlier outbreak in June–November in Nzara, Sudan, was initially thought to be linked, but was shown to have been caused by a different species of Ebola virus.

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Selected virus

Diagram of adenovirus

Adenoviruses are a group of non-enveloped DNA viruses that make up the Adenoviridae family. At 90–100 nm in diameter, they are the largest known viruses to lack an envelope. Unique spikes or fibres protrude from the icosahedral capsid, with knobs that bind to the receptor on the host cell. The linear double-stranded genome is 26 to 48 kb long, encodes 23 to 46 proteins, and has a 55 kDa protein attached to each end.

Adenoviruses infect a broad range of vertebrates, including humans, livestock, horses, dogs, bats and other mammals, as well as birds and reptiles, and usually cause infections of the upper respiratory tract. The virus is very stable. Transmission can occur by respiratory droplets or via faeces, with swimming pools being a common source of infection. A total of 57 serotypes, grouped into 7 species, have been found in humans; they cause a wide range of illnesses including mild respiratory infections, conjunctivitis and gastroenteritis. Some types have been associated with obesity. In people with an immunodeficiency they can cause life-threatening multi-organ disease. No antiviral treatment or vaccine is generally available; hand washing is the best way to prevent infection. Adenoviruses are an important viral vector for gene therapy. An oncolytic adenovirus has been approved in China for the treatment of head and neck cancer.

Did you know?

Electron micrograph of Bourbon virus

Selected biography

Ali Maow Maalin (1954 – 22 July 2013) was a hospital cook and health worker from Merca, Somalia, who is the last person in the world known to be infected with naturally occurring smallpox. Although he worked in the local smallpox eradication programme, he had not been successfully vaccinated. In October 1977, he was infected with the Variola minor strain of the virus while driving two children with smallpox symptoms to quarantine. He did not experience complications and made a full recovery. An aggressive containment campaign was successful in preventing an outbreak, and smallpox was declared to have been eradicated globally by the World Health Organization (WHO) two years later.

In later life, Maalin volunteered for the successful poliomyelitis eradication campaign in Somalia. He worked for WHO as a local coordinator with responsibility for social mobilisation, and spent several years travelling across Somalia, vaccinating children and educating communities. He encouraged people to be vaccinated by sharing his experiences with smallpox. He died of malaria while carrying out polio vaccinations after the reintroduction of poliovirus to the country in 2013.

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

Administration of an Ebola vaccine candidate in a clinical trial

The first Ebola vaccine was approved in 2019. Developed by the Public Health Agency of Canada, rVSV-ZEBOV is based on an attenuated recombinant vesicular stomatitis virus, genetically modified to express a surface glycoprotein of Zaire ebolavirus, and is estimated to be 97.5% effective. In the Kivu Ebola epidemic of 2018–20, a ring vaccination strategy was employed to protect direct and indirect contacts of infected people, as well as health workers, and around 300,000 people were vaccinated with rVSV-ZEBOV. A second vaccine was approved in 2020; this uses two different doses – a vector based on human adenovirus serotype 26 used to prime, boosted around eight weeks later by modified vaccinia Ankara (based on a heavily attenuated vaccinia virus) – and is not suitable for response to an outbreak. The efficacy is unknown. Multiple other vaccine candidates are in development to prevent Ebola, including replication-deficient adenovirus vectors, replication-competent human parainfluenza 3 vectors, and virus-like nanoparticle preparations.



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