The Viruses Portal
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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
Dengue fever, or breakbone fever, is an infectious tropical disease caused by the dengue virus. Symptoms include fever, headache, muscle and joint pains, and a characteristic skin rash that is similar to measles. Rarely, the disease develops into the life-threatening dengue haemorrhagic fever, resulting in bleeding, low levels of blood platelets and blood plasma leakage, or into dengue shock syndrome, where dangerously low blood pressure occurs.
Dengue is transmitted by several species of Aedes mosquito, mainly A. aegypti. First described in 1779, the incidence of dengue fever has increased dramatically since the Second World War, with around 390 million people infected each year, of whom about 40,000 die. The virus is endemic in more than 120 countries, mainly in Southeast Asia, South Asia and South America. Infection with one of the five viral types usually gives lifelong immunity to that type but only short-term immunity to the others, and subsequent infection with a different type increases the risk of severe complications. Treatment of acute dengue is supportive, using oral or intravenous fluids for mild or moderate disease; blood transfusion may be required in more severe cases. A vaccine against dengue fever is available. Reducing mosquito habitats and limiting exposure to bites are the other main methods of control.
Selected image
HIV-1 budding from lymphocytes in culture. HIV establishes a latent infection in several types of immune cell and causes profound immunodeficiency.
Credit: C. Goldsmith (1984)
In the news
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
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The Hershey–Chase experiments were conducted by Alfred Hershey and Martha Chase in 1952 using the T2 bacteriophage (pictured), which is composed of DNA wrapped in a protein shell. Hershey and Chase labelled either the phage DNA using radioactive phosphorus-32 or the protein using radioactive sulphur-35. They allowed the radiolabelled phages to infect unlabelled bacteria, and then agitated in a blender and centrifuged to separate material remaining outside the bacterial cells. The majority of the 32P-labelled DNA entered the host bacterial cell, while all the 35S-labelled protein remained outside. Hershey and Chase also showed that the phage DNA is inserted into the bacteria shortly after the virus attaches to its host.
DNA had been known since 1869, but in 1952 many scientists believed that proteins carried the information for inheritance. Proteins appeared more complex, while DNA was thought to be an inert molecule used for phosphorus storage. These experiments built on earlier research on transformation in bacteria and helped to confirm that DNA, not protein, was the genetic material. Hershey shared the 1969 Nobel Prize in Physiology or Medicine for the research.
Selected outbreak
The West African Ebola epidemic was the most widespread outbreak of the disease to date. Beginning in Meliandou in southern Guinea in December 2013, it spread to adjacent Liberia and Sierra Leone, affecting the cities of Conakry and Monrovia, with minor outbreaks in Mali and Nigeria. Cases reached a peak in October 2014 and the epidemic was under control by late 2015, although occasional cases continued to occur into April 2016. Ring vaccination with the then-experimental vaccine rVSV-ZEBOV was trialled in Guinea.
More than 28,000 suspected cases were reported with more than 11,000 deaths; the case fatality rate was around 40% overall and around 58% in hospitalised patients. Early in the epidemic nearly 10% of the dead were healthcare workers. The outbreak left about 17,000 survivors, many of whom reported long-lasting post-recovery symptoms. Extreme poverty, dysfunctional healthcare systems, distrust of government after years of armed conflict, local burial customs of washing the body, the unprecedented spread of Ebola to densely populated cities, and the delay in response of several months all contributed to the failure to control the epidemic.
Selected quotation
| “ | ...the variety of genes on the planet in viruses exceeds, or is likely to exceed, that in all of the rest of life combined. | ” |
Recommended articles
Viruses & Subviral agents: bat virome • elephant endotheliotropic herpesvirus • HIV • introduction to viruses
• Playa de Oro virus • poliovirus • prion • rotavirus • virus
Diseases: colony collapse disorder • common cold • croup • dengue fever • gastroenteritis • Guillain–Barré syndrome • hepatitis B • hepatitis C • hepatitis E • herpes simplex • HIV/AIDS • influenza • meningitis • myxomatosis • polio • pneumonia • shingles • smallpox
Epidemiology & Interventions: 2007 Bernard Matthews H5N1 outbreak • Coalition for Epidemic Preparedness Innovations • Disease X • 2009 flu pandemic • HIV/AIDS in Malawi • polio vaccine • Spanish flu • West African Ebola virus epidemic
Virus–Host interactions: antibody • host • immune system • parasitism • RNA interference
Methodology: metagenomics
Social & Media: And the Band Played On • Contagion • "Flu Season" • Frank's Cock • Race Against Time: Searching for Hope in AIDS-Ravaged Africa • social history of viruses • "Steve Burdick" • "The Time Is Now" • "What Lies Below"
People: Brownie Mary • Macfarlane Burnet • Bobbi Campbell • Aniru Conteh • people with hepatitis C • HIV-positive people • Bette Korber • Henrietta Lacks • Linda Laubenstein • Barbara McClintock • poliomyelitis survivors • Joseph Sonnabend • Eli Todd • Ryan White
Selected virus
Coronaviruses are a subfamily of RNA viruses in the Nidovirales order which infect mammals and birds. They are spherical enveloped viruses, generally around 80–120 nm in diameter, containing a helical nucleocapsid. Their positive-sense single-stranded RNA genome ranges from approximately 26 to 32 kb in size, one of the largest among RNA viruses. Around 74 characteristic club-shaped spikes project from the envelope, which in electron micrographs resemble the solar corona, from which their name derives. Infectious bronchitis virus was isolated in 1933 from chickens; two mice coronaviruses causing hepatitis and encephalomyelitis were discovered in the 1940s.
Coronaviruses predominantly infect epithelial cells, with the viral spike protein determining tissue tropism and host range. Animal coronaviruses often infect the gastrointestinal tract, causing diarrhoea in cows and pigs, and are transmitted by the faecal–oral route. Human and bird coronaviruses infect the respiratory tract, are transmitted via aerosols and droplets; they cause respiratory tract infections that can range from mild to lethal. Mild illnesses in humans include around 15% of common cold cases, while more lethal coronaviruses cause SARS, MERS and COVID-19. Many human coronaviruses have evolved from viruses of bats.
Did you know?
- ...that George Hirst was the first person to discover a viral enzyme (pictured)?
- ...that fish diseases and parasites can result in mass fish die offs?
- ...that adjuvants are sometimes used to modify the effects that a vaccine has on disease resistance by stimulating the immune system to respond to the vaccine with much more vitality?
- ...that Australian ophthalmologist Sir Norman McAlister Gregg discovered the link between rubella and congenital disorders in newborn infants after overhearing several of his patients discussing their illness during pregnancy?
- ...that the very first news article on what became known as AIDS appeared in the New York Native, a now defunct gay newspaper in New York City?
Selected biography
Thomas Flewett (29 June 1922 – 12 December 2006) was a British–Irish virologist and an authority on electron microscopy of viruses, best known for his role in the discovery of rotaviruses. After Ruth Bishop and others discovered viruses associated with diarrhoea, Flewett showed that they could be visualised by electron microscopy directly in faeces. He dubbed them "rotaviruses" for their wheel-shaped appearance. His group described the different rotavirus serotypes, and did extensive research on the rotavirus varieties infecting many animals.
Flewett established one of the first English virus laboratories in Birmingham in 1956. In addition to his rotavirus work, he discovered the cause of hand, foot and mouth disease, identified two new species of adenovirus, and co-discovered human torovirus and picobirnaviruses. His other research included influenza, coxsackie A, coxsackie B and hepatitis B viruses.
In this month
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
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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