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".
AIDS is a progressive immunodeficiency disease first identified in 1981, caused by infection with human immunodeficiency virus (HIV). Initial infection is sometimes associated with a brief influenza-like illness, followed by a period of 3–20 years without symptoms during which the immune system deteriorates. The individual becomes highly susceptible to common infections such as tuberculosis, as well as opportunistic infections, such as Pneumocystis pneumonia, and tumours, such as Kaposi's sarcoma. Severe weight loss is also a feature of AIDS. Without treatment, the average survival time after infection is around 9–11 years, depending on the HIV subtype.
HIV/AIDS is a global pandemic. As of 2012, around 35.3 million people worldwide are estimated to be infected with HIV, with 2.3 million new infections occurring each year. AIDS has caused around 36 million deaths. The major routes of transmission are unsafe heterosexual or male/male sex, contaminated needles and from mother to child, either at birth or via breastfeeding. Combination antiretroviral therapy does not eliminate the virus, but delays progression to AIDS and can lead to a near-normal life expectancy. Adverse effects of therapy remain a problem. Drugs to prevent infection have recently been licensed, but there is no effective HIV vaccine.
A global drive to eradicate poliovirus started in 1988, when there were an estimated 350,000 cases of wild poliovirus infection globally. Two diseases, both caused by viruses, have been eradicated, smallpox in 1980 and rinderpest in 2011. Poliovirus only infects humans. It persists in the environment for a few weeks at room temperature and a few months at 0–8 °C. The oral polio vaccine is inexpensive, highly effective and is predicted to generate lifelong immunity.
A lack of basic health infrastructure and civil war remain significant obstacles to eradication. Some local communities have opposed immunisation campaigns, and vaccination workers have been murdered in Pakistan and Nigeria. A total of 72 cases of wild poliovirus infection were reported during 2015 – 53 in Pakistan and 19 in Afghanistan – and as of 2016, poliovirus is endemic in only those two countries and Nigeria. Reversion of live vaccine strains to virulence has resulted in occasional cases of vaccine-associated paralysis.
In the news
23 May: An outbreak of measles (virus pictured) is ongoing in the Amazonas and Roraima states of Brazil, with 995 suspected cases, including two deaths. WHO
21 May: Vaccination with rVSV-ZEBOV starts in the Ebola virus outbreak in the Democratic Republic of the Congo; the outbreak has spread from rural Bikoro to the city of Mbandaka, with a total of 58 suspected cases reported, including 27 deaths. WHO
21 May: An outbreak of Nipah virus has been confirmed in Kozhikode, Kerala state, south India. BBC
17 May: The sialic acid-bearing cellular receptor for influenza A virus is shown to be the voltage-gated calcium channel, Cav1.2 (pictured). Cell Host & Microbe
16 May: The cellular receptor for several alphaviruses associated with rheumatic disease, including chikungunya, Mayaro, O'nyong nyong and Ross River viruses, is shown to be the cell adhesion molecule Mxra8. Nature
14 May: IMP-1088, a compound targeting human N-myristoyltransferases NMT1 and NMT2, is shown to prevent three different picornaviruses, rhinovirus, poliovirus and foot-and-mouth disease virus, from assembling capsids in vitro, by inhibiting myristoylation of the picornaviral VP0 protein. Nat Chem
9 May: Hepatitis B virus sequences are recovered from Bronze Age human remains up to 4,500 years old, and the virus is estimated to have evolved 8,600–20,900 years ago, disproving the hypothesis that it originated in the New World and spread to Europe in around the 16th century. Nature
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).
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.
||The unusual features of the giant Mimivirus revived the popular, yet unresolved question: "Are viruses alive?" The discovery that some of them can get sick adds a new twist to this old debate.
Rotavirus is a genus of double-stranded RNA viruses in the family Reoviridae. There are five species A–E; rotavirus A, the most common, causes over 90% of infections in humans. Rotavirus also infects animals, including livestock. The virus is transmitted by the faecal–oral route, with fewer than 100 virus particles being required for infection. Rotaviruses are stable in the environment and normal sanitary measures fail to protect against them. Effective rotavirus vaccines are the main prevention method.
The virus infects and damages the enterocytes lining the small intestine, causing gastroenteritis (sometimes referred to as "stomach flu," although the virus is not related to influenza). A viral toxin is responsible for some of the pathology. Rotavirus is the most common cause of severe diarrhoea among infants and young children. Almost every child worldwide has been infected with rotavirus at least once by the age of five. Over 500,000 children under five die from rotavirus infection each year and almost two million more become severely ill. Immunity develops with repeated infections and adults are rarely affected.
Did you know?
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
Aciclovir (also acyclovir, Zovirax) is a nucleoside analogue, which mimics the nucleoside guanosine. After phosphorylation by viral thymidine kinase and cellular enzymes, it 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 Schaffer and colleagues, and developed by Schaffer 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. One of the most commonly used antiviral drugs, aciclovir is active against most viruses in the herpesvirus family. It is mainly used to treat herpes simplex virus infections, chickenpox and shingles. Aciclovir resistance is rare.