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
Rabies is a disease of humans and some other mammals, generally caused by the rabies virus, an RNA virus in the Rhabdoviridae family. A few cases have involved the closely related Australian bat lyssavirus. Rabies virus has a wide host range. Globally, dogs are the main source of human infections, with bats being important in the Americas; other naturally infected animals include monkeys, raccoons, foxes, skunks, cattle, horses, wolves, coyotes, cats, mongooses, bears, groundhogs, weasels and other carnivores. Transmission is commonly via saliva, usually but not always from bites; it can potentially occur via aerosols contacting mucous membranes. The typical human incubation period is 1–3 months. The neurotropic virus travels along neural pathways into the CNS and brain, where it causes meningoencephalitis. Nonspecific symptoms such as fever and headache are followed by neurological symptoms, including partial paralysis, confusion, agitation, paranoia, hallucinations and sometimes hydrophobia, which progress to delirium, coma and death. Around 17,400 people died from rabies in 2015, mainly in Asia and Africa.
Rabies is mentioned in the Codex of Eshnunna of around 1930 BC. The first vaccine was developed in 1885 by Louis Pasteur and Émile Roux. Prophylactic vaccination is used in people at high risk, pets and wild animals. Post-exposure prophylaxis, including vaccine and immunoglobulin, is completely effective if begun immediately after exposure, but survival is extremely rare once symptoms have begun.
Selected image
Ebola virus is a filamentous RNA virus first recognised in 1976. Four of the five known members of the Ebolavirus genus cause a severe haemorrhagic fever in humans.
Credit: Cynthia Goldsmith
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
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 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
| “ | A virus is a piece of bad news wrapped in protein. | ” |
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 Australian pilot and nurse Robin Miller (monument pictured) borrowed money to buy a Cessna 182 and flew to remote outback areas in Western Australia to vaccinate Indigenous Australian children against polio, becoming known as the "Sugarbird Lady"?
- ...that the Mokola virus is a relative of the rabies virus and was first isolated in tree shrews?
- ...that in 1995–97, a Centers for Disease Control program identified over a hundred patients likely suffering from life-threatening infections new in the history of emerging infectious diseases?
- ...that Batheay District in Kampong Cham Province, Cambodia, was the site of an outbreak of the deadly H5N1 virus in 2006?
- ...that Fig Trees, an operatic documentary about AIDS activism, is narrated by a singing albino squirrel?
Selected biography
Frederick Sanger (13 August 1918 – 19 November 2013) was a British biochemist, the only person to have been awarded the Nobel Prize for Chemistry twice.
He started to research nucleic acid sequencing in the early 1960s. In 1975, he co-invented the "Plus and Minus" technique for sequencing DNA, which could sequence 80 nucleotides at once, a significant improvement on earlier techniques. Using this method, his group sequenced most of the 5,386 nucleotides of φX174 bacteriophage – the first virus and the first DNA genome to be completely sequenced – and showed that some of its genes overlapped. In 1977, he and his group pioneered the Sanger (or dideoxy chain-termination) method for sequencing DNA and used it to sequence the 48,502 bp bacteriophage λ. His technique remained the most widely used sequencing method until the mid-2000s, and was used to generate the first human genome sequence. Sanger is also known for sequencing bovine insulin, the first protein to be sequenced. The Sanger Institute was named for him.
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
Aciclovir (also acyclovir and sold as Zovirax) is a nucleoside analogue that mimics the nucleoside guanosine. It is active against most viruses in the herpesvirus family, and is mainly used to treat herpes simplex virus infections, chickenpox and shingles. After phosphorylation by viral thymidine kinase and cellular enzymes, the drug inhibits the viral DNA polymerase. Extremely selective and low in cytotoxicity, it was seen as the start of a new era in antiviral therapy. Aciclovir was discovered by Howard Schaeffer and colleagues, and developed by Schaeffer and Gertrude Elion, who was awarded the 1988 Nobel Prize in Medicine in part for its development. Nucleosides isolated from a Caribbean sponge, Cryptotethya crypta, formed the basis for its synthesis. Aciclovir differs from earlier nucleoside analogues in containing only a partial nucleoside structure: the sugar ring is replaced with an open chain. Resistance to the drug is rare in people with a normal immune system.
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