Portal:Viruses
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
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.
Selected image
Muhammad ibn Zakariya al-Razi was a Persian physician and chemist who, in the 9th century, was the first to document the distinction between the diseases of measles and smallpox.
Credit: Gerard of Cremona (c. 1250–60)
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 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.
Selected quotation
| “ | To help conceptualize the sheer number of viruses in existence, their current biomass has been estimated to equal that of 75 million blue whales (approximately 200 million tonnes) and, if placed end to end, the collective length of their virions would span 65 galaxies. | ” |
—Peter Simmonds
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
Epstein–Barr virus (EBV) (also human herpesvirus 4) is a DNA virus in the Herpesviridae family which infects humans. The virion is around 122–180 nm in diameter. As in all herpesviruses, the nucleocapsid is surrounded by a protein tegument, as well as an envelope. The double-stranded DNA genome is about 172 kb, with 85 genes, making it one of the more complex viruses.
Transmission is via saliva and genital secretions. The virus infects epithelial cells in the pharynx and B cells of the immune system, producing virions by budding. EBV also becomes latent in B cells, possibly in the bone marrow, allowing the infection to persist lifelong. In the latent state, the linear genome is made circular and replicates in the nucleus separately from the host DNA as an episome. Reactivation is poorly understood but is thought to be triggered by the B cell responding to other infections. EBV infection occurs in around 95% of people. Infectious mononucleosis or glandular fever can occur when first infection is delayed until adolescence or adulthood. EBV is associated with some types of cancer, including Burkitt's lymphoma and nasopharyngeal carcinoma. In people with HIV, it can cause hairy leukoplakia and central nervous system lymphomas.
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
Françoise Barré-Sinoussi (born 30 July 1947) is a French virologist, known for being one of the researchers who discovered HIV.
Barré-Sinoussi researched retroviruses in Luc Montagnier's group at the Institut Pasteur in Paris. In 1982, she and her co-workers started to analyse samples from people with a new disease, then referred to as "gay-related immune deficiency". They found a novel retrovirus in lymph node tissue, which they called "lymphadenopathy-associated virus". Their results were published simultaneously with those of Robert Gallo's group in the USA, who had independently discovered the virus under the name "human T-lymphotropic virus type III". The virus, renamed "human immunodeficiency virus", was later shown to cause AIDS. Barré-Sinoussi continued to research HIV until her retirement in 2015, studying how the virus is transmitted from mother to child, the immune response to HIV, and how a small proportion of infected individuals, termed "long-term nonprogressors", can limit HIV replication without treatment. In 2008, she was awarded the Nobel Prize, with Montagnier, for the discovery of HIV.
In this month
5 June 1981: First report of HIV/AIDS (symbol pictured) appeared in medical literature
6 June 1997: Gene silencing in plants shown to be a viral defence mechanism
7–13 June 1962: Donald Caspar and Aaron Klug proposed the quasi-equivalence principle of virus structure
7–13 June 1962: André Lwoff proposed a viral classification scheme based on nature of genome, type of symmetry and presence of envelope
7–13 June 1962: George Hirst proposed that the influenza virus genome is segmented
9 June 1981: The American Society for Virology was founded
13 June 2012: First case of Middle East respiratory syndrome coronavirus (MERS-CoV) occurred in Saudi Arabia
18 June 1981: A vaccine against foot-and-mouth disease was the first genetically engineered vaccine
21 June 1996: Nevirapine approved, first NNRTI for HIV/AIDS
26 June 1993: Clinical trial of hepatitis B virus drug fialuridine terminated; the drug caused several fatalities due to lactic acidosis
28 June 2011: FAO declared rinderpest eradicated
30 June 1985: Ryan White was denied re-admittance to his school after an AIDS diagnosis, in a case that changed public perceptions of the disease
Selected intervention
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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