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 5,000 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
Meningitis is an acute inflammation of the meninges, protective membranes covering the brain and spinal cord. Symptoms in adults include headache, fever and neck stiffness, as well as sometimes confusion or altered consciousness, vomiting, and an inability to tolerate light or loud noises. Children often show only nonspecific symptoms, such as irritability or drowsiness.
The most common cause is infection with viruses including enteroviruses, herpes simplex virus (mainly HSV-2), varicella zoster virus, mumps virus, HIV and lymphocytic choriomeningitis. In Western countries, viral meningitis occurs in around 11 people per 100,000 each year. Infection with bacteria, fungi, protozoa and parasites can also cause meningitis, and there are several non-infectious causes. Although some forms of meningitis can be life-threatening, viral meningitis is generally more benign than that caused by bacterial infection. It usually resolves spontaneously and is rarely fatal. HSV-2 can cause a chronic, recurrent form called Mollaret's meningitis.
Polymerase chain reaction of cerebrospinal fluid and identification of antibodies can be used to differentiate between viral causes. Viral meningitis typically only requires supportive therapy; meningitis caused by HSV or varicella zoster virus sometimes responds to treatment with antiviral drugs such as aciclovir. Mumps-associated meningitis can be prevented by vaccination.
Selected image
The striping caused by tulip breaking virus, first described in 1576 by Carolus Clusius, was the second plant virus disease to be documented. The effects were much prized by 17th-century tulip growers.
Credit: Unknown (before 1640)
In the news
4 April: The ongoing pandemic of a novel coronavirus is accelerating rapidly; more than a million confirmed cases, including more than 57,000 deaths, have been documented globally since the outbreak began in December 2019. WHO 1, 2
27 March: An international, randomised, non-blinded, clinical trial organised by the World Health Organization of four potential treatments for COVID-19 – remdesivir; chloroquine or hydroxychloroquine; lopinavir/ritonavir; or lopinavir/ritonavir plus interferon-beta – is about to start enrolling patients. Science, WHO
16 March: A phase I clinical trial of a messenger RNA-based vaccine candidate for the novel coronavirus begins in Seattle. NIH
11 March: The World Health Organization describes the ongoing outbreak of respiratory disease caused by a novel coronavirus as a pandemic. WHO
10 March: A patient with apparent clearance of HIV after stem-cell therapy continues to have no viable virus detectable in blood or other reservoirs after 30 months without antiretroviral treatment. Lancet
9 March: No new cases have been recorded in three weeks in the ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo; as of 3 March there had been a total of 3444 cases, including 2264 deaths, since the outbreak began in August 2018. WHO 1, 2
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12 February: The ongoing Ebola virus outbreak in the North Kivu and Ituri provinces of the Democratic Republic of the Congo remains a Public Health Emergency of International Concern, according to the World Health Organization. WHO 1
7 February: Chinese scientists announce that novel coronavirus (2019-nCoV) is 99% identical to a coronavirus isolated from pangolins, suggesting these animals might be an intermediate host. Nature
5 February: A study of 2658 samples from 38 different types of cancer found that 16% were associated with a virus, higher than previous estimates, but did not identify any new candidate tumour viruses. Nat Genet
4 February: Over 2500 putative circular DNA virus genomes are catalogued from metagenomic surveys of human and animal samples, including over 600 dissimilar to existing virus groups. eLife, Science
3 February: The US National Institute of Allergy and Infectious Diseases stops the South African HVTN 702 Phase IIb/III clinical trial of an investigational HIV vaccine early, after the vaccine failed to prevent HIV infection. NIH
Selected article
Virus quantification is necessary for viral vaccine production, and is used to manage people infected with HIV, hepatitis B and C, and cytomegalovirus. A wide range of traditional and modern methods are used. Plaque assays (pictured) infect a monolayer of host cells with dilutions of the virus and count the number of holes or plaques, where cells have lysed and infected their neighbours. For viruses that do not lyse their host cell, plaques of cells showing cytopathic effects can be counted or viral proteins can be immunostained with fluorescent-labelled antibodies. These methods quantify infectious virus, while others, such as electron microscopy, return a higher concentration because they count all virus particles, whether or not they are viable. Other assays, such as the haemagglutinin assay, quantify viral proteins.
Often slow and labour intensive, traditional methods have been complemented by modern technologies that greatly reduce quantification time, including quantitative polymerase chain reaction, flow cytometry, enzyme-linked immunosorbent assays and tunable resistive pulse sensing.
Selected outbreak
The last recorded smallpox death occurred during the 1978 smallpox outbreak in Birmingham, UK. The outbreak resulted from accidental exposure to the Abid strain of Variola major, from a laboratory, headed by Henry Bedson, at the University of Birmingham Medical School – also associated with an outbreak in 1966. Bedson was investigating strains of smallpox known as whitepox, considered a potential threat to the smallpox eradication campaign, then in its final stages.
A medical photographer who worked on the floor above the laboratory showed smallpox symptoms in August and died the following month; one of her contacts was also infected but survived. The government inquiry into the outbreak concluded that she had been infected in late July, possibly via ducting, although the precise route of transmission was subsequently challenged. The inquiry criticised the university's safety procedures. Bedson committed suicide while under quarantine. Radical changes in UK research practices for handling dangerous pathogens followed, and all known stocks of smallpox virus were concentrated in two laboratories.
Selected quotation
| “ | A virus is a piece of bad news wrapped in protein. | ” |
Recommended articles
Viruses & Subviral agents: 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 • poliomyelitis • pneumonia • shingles • smallpox
Epidemiology & Interventions: 2007 Bernard Matthews H5N1 outbreak • 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"
People: Brownie Mary • Macfarlane Burnet • 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
Adeno-associated viruses (AAVs) are two small DNA viruses in the Dependoparvovirus genus of the Parvoviridae family. They cannot complete their lytic replication cycle without a helper virus, which include adenoviruses, herpesviruses and vaccinia. In the absence of the helper, AAVs can integrate into the host genome at a specific site on human chromosome 19, or persist as an episome. The 20 nm icosahedral capsid lacks an envelope, and contains a single-stranded DNA genome of around 4.7 kb. AAVs infect humans and some other primates without causing disease. They generate only a mild immune response, including neutralising antibodies. The best-studied of the 11 serotypes, AAV-2, infects nerve cells, liver cells, skeletal muscle and vascular smooth muscle, using heparan sulphate proteoglycan as its primary receptor.
Its low pathogenicity makes AAV an attractive basis for viral vectors for gene therapy. Alipogene tiparvovec to treat lipoprotein lipase deficiency was the first gene therapy to be licensed, but was later withdrawn. Promising results have been obtained in early clinical trials with AAV-based gene therapy in haemophilia, congestive heart failure, spinal muscular atrophy, Parkinson's disease and the rare eye disease Leber congenital amaurosis.
Did you know?
- ...that the genetically modified plum C5 is the only Prunus species resistant to the devastating plant disease plum pox?
- ...that CCR5 receptor antagonists are drugs that inhibit the entry of HIV into cells of the immune system?
- ...that during the 2008–09 flu season in the United States, only 14.1% of influenza tests were positive for influenza?
- ...that research by Dorothy M. Horstmann and Robert W. McCollum at Yale University showed that the poliovirus reaches the brain by way of the blood, leading to the development of the polio vaccine in the early 1950s?
- ...that sea sponge aquaculture has the potential to generate novel anti-inflammatory and antiviral drugs?
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
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
Oseltamivir (also Tamiflu) is an oral antiviral drug against influenza (flu). It was the second inhibitor of the viral neuraminidase to be developed, after zanamivir, and the first to be taken as an oral tablet. It was originally synthesised from shikimic acid extracted from the star anise plant. Oseltamivir is a prodrug that requires metabolism in the liver to the active form, oseltamivir carboxylate. This binds at the active site of the neuraminidase enzyme, preventing it from cleaving sialic acid to release the virus particle from the host cell. Oseltamivir can reduce the duration of influenza symptoms by 0.5–1 days. Debate is ongoing about whether it also reduces the risk of complications, such as pneumonia. Nausea and vomiting are the main adverse events. Resistance to oseltamivir has been observed in some strains of influenza virus, especially H1N1 strains.
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