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
The common cold is an upper respiratory tract disease that mainly affects the nose, and sometimes the throat, larynx and sinuses. Over 200 viruses can cause colds, most commonly rhinoviruses but also coronaviruses, influenza viruses, adenoviruses and others. Adults catch an average of 2–3 colds a year and children 6–8, making it the most common infectious human disease. The economic costs are huge, with colds responsible for 40% of time lost from work in the U.S. Colds are described in the Egyptian Ebers papyrus, the oldest surviving medical text, written before the 16th century BCE.
Symptoms include cough, sore throat, runny nose, nasal congestion, sneezing and sometimes muscle aches and headache; fever is common in young children. Symptoms typically resolve in 7–10 days, although some can last up to 3 weeks. The immune response to infection, rather than tissue destruction by the virus, causes most of the symptoms. Transmission occurs via airborne droplets and by contact with nasal secretions or contaminated objects. The viruses that cause colds can survive for prolonged periods in the environment (over 18 hours for rhinoviruses). Hand washing can help to prevent spread. No effective antiviral treatment or vaccine currently exists.
Selected image
1802 cartoon of Edward Jenner administering cowpox vaccine against smallpox, satirising contemporary fears about vaccination.
Credit: James Gillray (12 June 1802)
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
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.
Selected 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; pictured).
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
“ | Some scientists visualize the virus as an ill-defined shape emerging bashfully out of a dense and golden cloud. This is a beautiful and romantic vision. Virology should, however, not be too Turnerian. Nor should it be an abstract art. The portrait of a virus should not produce an aesthetic emotion by means of an organic disturbance. | ” |
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
Human immunodeficiency virus (HIV) is a lentivirus, an RNA virus in the retrovirus family. Two types of HIV have been characterised: HIV-1 is the more virulent and is responsible for most infections worldwide; HIV-2 is mainly confined to West Africa. The genome consists of two copies of a single-stranded +RNA, which contains nine genes. The roughly spherical virus particle has a diameter of about 120 nm; it is enveloped and contains a conical capsid made of around 2,000 copies of the p24 protein. The envelope glycoprotein, a trimeric complex of gp120 and gp41, binds to CD4, the primary receptor on the host cell.
Transmission occurs by the transfer of bodily fluids including blood, semen, vaginal fluids and breast milk, in which the virus is present both as free virus particles and within infected immune cells. HIV infects key cells in the human immune system including CD4+ T helper cells, macrophages and dendritic cells. Infection leads to low levels of CD4+ T cells via several mechanisms, resulting in a progressive immunodeficiency disease known as AIDS.
Did you know?
- ...that murine polyomavirus (pictured) is an oncovirus that causes tumours in newborn mice?
- ...that Pat Nuttall showed that systemic infection of the host is not required for pathogens such as tick-borne encephalitis virus to be transmitted between vectors?
- ...that the first person to die in Australia from the 2009 flu was a Pintupi man whose people gave up hunting to settle the remote community of Kiwirrkura at the time of his birth?
- ...that the 2011 film Asmaa is the first feature-length Egyptian drama film to present AIDS patients sympathetically?
- ...that the anti-vaccine book Melanie's Marvelous Measles has received over 1,000 one-star reviews on Amazon.com?
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
March 1990: Proposal for a database of all viruses, later the ICTVdB
3 March 2014: Discovery of Pithovirus sibericum, the largest known virus at 1.5 μm long by 0.5 μm in diameter
4 March 1918: First case reported in the 1918 influenza pandemic
10 March 1956: Francis Crick and James Watson proposed that small viruses have a protein shell consisting of a large number of identical subunits
10 March 1956: Donald Caspar published paper on the structure of tomato bushy stunt virus
13 March 2003: Enfuvirtide (T20) approved; first HIV fusion inhibitor, also first HIV entry inhibitor
20 March 1987: Antiretroviral drug AZT (pictured) became the first antiviral medication approved for use against HIV/AIDS
22 March 2014: First case reported in the West African Ebola outbreak, the most widespread so far
26 March 1953: Jonas Salk reported a successful test of an inactivated polio vaccine.
28 March 2003: Mimivirus shown to be a virus, then the largest known
Selected intervention
Ribavirin is a nucleoside analogue that mimics the nucleoside guanosine. It shows some activity against a broad range of DNA and RNA viruses, but is less effective against dengue fever, yellow fever and other flaviviruses. The drug was first synthesised in the early 1970s by Joseph T. Witkowski and Roland K. Robins. Ribavirin's main current use is against hepatitis C, in combination with pegylated interferon, nucleotide analogues and protease inhibitors. It has been used in the past in an aerosol formulation against respiratory syncytial virus-related diseases in children. Ribavirin has been used in combination as part of an experimental treatment for rabies. It is also the only available treatment for the viruses causing some viral haemorrhagic fevers, including Lassa fever, Crimean–Congo haemorrhagic fever and hantavirus disease, but is ineffective against the filovirus diseases, Ebola and Marburg. Clinical use is limited by the drug building up in red blood cells to cause haemolytic anaemia.
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