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 deformed right leg due to poliomyelitis

Polio, also called poliomyelitis or infantile paralysis, was one of the most feared childhood diseases of the 20th century. Poliovirus, the causative agent, only naturally infects humans and spreads via the faecal–oral route. Most infections cause no or minor symptoms. In around 1% of cases, the virus enters the central nervous system, causing aseptic meningitis. There it can preferentially infect and destroy motor neurons, leading in 0.1–0.5% of cases to muscle weakness and acute flaccid paralysis. Spinal polio accounts for nearly 80% of paralytic cases, with asymmetric paralysis of the legs being typical; in a quarter of these cases permanent severe disability results. Bulbar involvement is rare, but in severe cases the virus can prevent breathing by affecting the phrenic nerve, so that patients require mechanical ventilation with an iron lung or similar device.

Depictions in ancient art show that the disease has existed for thousands of years. The virus was an endemic pathogen until the 1880s, when major epidemics began to occur in Europe and later the United States. Polio vaccines were developed in the 1950s and a global eradication campaign started in 1988. The annual incidence of wild-type disease fell from many hundreds of thousands to 22 in 2017, but has since resurged to a few hundreds.

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Castor bean tick, Ixodes ricinus

The castor bean tick, Ixodes ricinus, can transmit the tick-borne encephalitis virus. Ticks are common vectors for viruses, and other tick-borne diseases include Colorado tick fever and Crimean–Congo haemorrhagic fever.

Credit: Richard Bartz (24 April 2009)

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

22 November: In the ongoing pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), nearly 58 million confirmed cases, including more than 1.3 million deaths, have been documented globally since the outbreak began in December 2019. 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

16 November: Interim unpublished results from an ongoing Phase III trial in around 30,000 participants suggest that an mRNA vaccine from Moderna targeting the SARS-CoV-2 S glycoprotein is at least 94% effective at preventing COVID-19 disease after two doses. Nature

9 November: Interim unpublished results from an ongoing Phase III trial in 43,538 participants suggest that an mRNA vaccine from Pfizer/BioNTech targeting the SARS-CoV-2 S glycoprotein is at least 90% effective at preventing COVID-19 disease after two doses. Nature

5 November: A cluster of COVID-19 cases associated with mink farming, in which the causative SARS-CoV-2 strain harbours a novel combination of mutations and appears less susceptible to neutralising antibodies, are reported from North Jutland, Denmark; the news triggers the slaughter of all farmed mink across the country. WHO

29 October: The Howard Hughes Medical Institute announces a donation to the Henrietta Lacks Foundation, intended as reparation for the harvesting of cervical cancer cells from Lacks in 1951, without her consent; the cells were subsequently used to create the HeLa cell line. Nature

22 October: The US FDA approves the repurposed antiviral remdesivir for the treatment of SARS-CoV-2 infection; the decision has been criticised owing to the drug's apparent lack of benefit in as-yet-unpublished interim results from the World Health Organization's (WHO) Solidarity trial. Science

14 October: A combination of three monoclonal antibodies, atoltivimab, maftivimab and odesivimab-ebgn, is licensed by the US FDA as the first treatment for Ebola virus disease. FDA

25 August: Africa is declared free from wild poliovirus, with the last case having been detected in Nigeria in 2016; vaccine-derived polio cases continue to be recorded in the region. Nature

20 August: The International Committee on Taxonomy of Viruses creates three new realms for DNA virusesDuplodnaviria, for double-stranded DNA (dsDNA) viruses with HK97-fold capsid proteins, including tailed bacteriophages and herpesviruses; Varidnaviria for dsDNA viruses with jelly-roll capsid proteins, such as adenoviruses and poxviruses; and Monodnaviria for most single-stranded DNA viruses, including papillomaviruses – and also extends Riboviria to include RNA viruses that replicate using reverse transcriptase. ICTV

Selected article

Baltimore classification

Virus classification is the process of naming viruses and placing them into a taxonomic system. They are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms and the type of disease they cause.

Two schemes are in common use. The International Committee on Taxonomy of Viruses (ICTV), established in the early 1970s, classifies viruses into taxa (groups) similar to the biological classification used for cellular organisms, which reflect viruses believed to have a common ancestor. As of 2019, 9 kingdoms, 16 phyla, 36 classes, 55 orders, 168 families, 1,421 genera and 6,589 species of viruses have been defined. Since 2018, viruses have also been classified into higher-level taxa called realms. Four realms are defined, as of 2020, encompassing almost all RNA viruses; some DNA viruses have yet to be assigned a realm.

The older Baltimore classification (pictured), proposed in 1971 by David Baltimore, places viruses into seven groups (I–VII) based on their nucleic acid type, number of strands and sense, as well as the method the virus uses to generate mRNA. There is some concordance between Baltimore groups and the higher levels of the ICTV scheme.

Selected outbreak

Passengers in Mexico City wearing face masks in an attempt to prevent infection

The 2009 flu pandemic was an influenza pandemic first recognised in Mexico City in March 2009 and declared over in August 2010. It involved a novel strain of H1N1 influenza virus with genes from five different viruses, which resulted when a previous triple reassortment of avian, swine and human influenza viruses further combined with a Eurasian swine influenza virus, leading to the term "swine flu" being used for the pandemic. It was the second pandemic to involve an H1N1 strain, the first being the 1918 "Spanish flu" pandemic.

The global infection rate was estimated as 11–21%. This pandemic strain was less lethal than previous ones, killing about 0.01–0.03% of those infected, compared with 2–3% for Spanish flu. Most experts agree that at least 284,500 people died, mainly in Africa and Southeast Asia – comparable with the normal seasonal influenza fatalities of 290,000–650,000 – leading to claims that the World Health Organization had exaggerated the danger.

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Selected virus

Cryo-electron microscopy image of Semliki Forest virus, an alphavirus

Alphaviruses are a genus of RNA viruses in the Togaviridae family. The spherical enveloped virion is 70 nm in diameter, with a nucleocapsid of 40 nm. It has a single-stranded, positive-sense RNA genome of 11–12 kb. The genus contains more than thirty species, which infect humans, horses, rodents and other mammals, as well as fish, birds, other vertebrates and invertebrates. Alphaviruses are generally transmitted by insect vectors, predominantly mosquitoes, and are an example of arboviruses (arthropod-borne viruses).

The first alphavirus to be discovered was western equine encephalitis virus, by Karl Friedrich Meyer in 1930, in horses with fatal encephalitis in San Joaquin Valley, California, USA. Some members of the genus cause significant disease in humans, including the chikungunya, o'nyong'nyong, Ross River, Sindbis, Barmah Forest and Semliki Forest (pictured) viruses and the eastern, western and Venezuelan equine encephalitis viruses. Arthritis, encephalitis, rashes and fever are the most frequently observed symptoms. Large mammals such as humans usually form dead-end hosts for the viruses, although Venezuelan equine encephalitis virus is mainly amplified in the horse. No human vaccine or antiviral drug has been licensed. Prevention is predominantly by control of the insect vector.

Did you know?

Erica Schwartz in 2011

Selected biography

Rosalind Elsie Franklin (25 July 1920 – 16 April 1958) was a British biophysicist and X-ray crystallographer who made critical contributions to the understanding of the fine molecular structures of DNA, RNA and viruses.

Franklin led pioneering research on the structure of tobacco mosaic virus (TMV), a rod-like RNA virus, using X-ray crystallography. She first showed that, contrary to contemporary opinion, TMV virus particles were all of the same length. With Kenneth Holmes, she showed the virus's coat is composed of protein molecules arranged in helices. She designed and built a model of the virus to be exhibited at the 1958 World's Fair. She speculated that the virus is hollow, and correctly hypothesized that the RNA of TMV is single-stranded. Her work, together with that of Donald Caspar, revealed that the viral RNA is wound along the inner surface of the hollow virus. Her laboratory, which also included Aaron Klug, studied other plant viruses, including turnip yellow mosaic virus and viruses infecting potato, tomato and pea. Franklin also worked on icosahedral animal viruses, including poliovirus.

Franklin is commemorated in the Rosalind Franklin University of Medicine and Science.

In this month

Electron micrograph of SARS coronaviruses

7 November 1991: Magic Johnson announced his retirement from basketball because of his infection with HIV

14 November 1957: Kuru, the first human prion disease, described by Daniel Gajdusek and Vincent Zigas

16 November 2002: The first case of severe acute respiratory syndrome (virus pictured) recorded in Guangdong, China

17 November 1995: Lamivudine approved for treatment of HIV

22 November 2013: Simeprevir approved for treatment of chronic hepatitis C virus infection

23 November 1978: Structure of tomato bushy stunt virus solved by Stephen Harrison and colleagues, the first atomic-level structure of a virus

24 November 2007: Outbreak of new Ebola species, Bundibugyo virus

26 November 1898: Martinus Beijerinck coined the term contagium vivum fluidum to describe the agent causing tobacco mosaic disease

Selected intervention

Ball-and-stick model of nevirapine

Nevirapine (also Viramune) is an antiretroviral drug used in the treatment of HIV/AIDS caused by HIV-1. It was the first non-nucleoside reverse transcriptase inhibitor to be licensed, which occurred in 1996. Like nucleoside inhibitors, nevirapine inhibits HIV's reverse transcriptase enzyme, which copies the viral RNA into DNA and is essential for its replication. Unlike nucleoside inhibitors, it binds not in the enzyme's active site but in a nearby hydrophobic pocket, causing a conformational change in the enzyme that prevents it from functioning. Mutations in the pocket generate resistance to nevirapine, which develops rapidly unless viral replication is completely suppressed. The drug is therefore only used together with other anti-HIV drugs in combination therapy. The HIV-2 reverse transcriptase has a different pocket structure, rendering it inherently resistant to nevirapine and other first-generation NNRTIs. A single dose of nevirapine is a cost-effective way to reduce mother-to-child transmission of HIV, and has been recommended by the World Health Organization for use in resource-poor settings. Other protocols are recommended in the United States. Rash is the most common adverse event associated with the drug.


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