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".
Cervical cancer is a tumour of the cervix, the junction between the uterus and vagina in the female reproductive tract. Certain types of human papillomavirus (HPV) are implicated in more than 90% of these cancers, although the great majority of HPV infections of the cervix are not associated with cancer. HPV is transmitted by vaginal sex, infecting cervical epithelial cells. In 5–10% of cases, infection persists for years, and pre-cancerous changes called cervical intraepithelial neoplasia can develop. These can regress, but sometimes progress to cancer. Although in nearly all forms of the cancer, HPV infection is considered essential for cancer to develop, other risk factors are involved, including smoking, HIV infection and other forms of immune suppression.
Cervical cancer is the fourth most common cancer in women worldwide. It can be detected by screening; screening every 3–5 years, with appropriate follow-up, can reduce cancer incidence by up to 80%. HPV vaccines protect against types 16 and 18, which cause three-quarters of cancers. Where screening and vaccination are not available, cervical cancer has substantial mortality; worldwide, an estimated 528,000 cases and 266,000 deaths occurred in 2012, with 80% of these being in developing countries.
In the news
18 January: The Coalition for Epidemic Preparedness Innovations commits $460 million to fast-tracking the development of vaccines against MERS coronavirus, Lassa virus and Nipah virus. BBC
18 November: WHO declares that Zika virus transmission and associated conditions (virus pictured) are a long-term situation which no longer qualifies as a Public Health Emergency of International Concern. WHO
4 October: Capsid protein assemblies of beak and feather disease virus, which infects endangered parrot species (infected bird pictured), are visualised by X-ray crystallography. Nat Commun
29 September: Zika virus is shown to infect the neural crest cells that develop into the cranium, driving cell death of neural progenitor cells by their impaired cytokine signalling. Cell Host Microbe
28 September: Ranaviruses (pictured), which cause severe disease in wild amphibians, are found to be spread in the UK by human activities. Proc R Soc London B
27 September: Rift Valley fever virus infection during pregnancy substantially increased the risk of miscarriage in a cross-sectional study in Sudan. Lancet Global Health
27 September: WHO declares measles to have been eliminated from North and South America. PAHO/WHO
26 September: Broadly neutralising antibodies to HIV are found in 239 HIV+ participants in the Swiss Cohort Study, with a significantly higher rate among black people. Nat Med
15 September: In a novel mouse model of hepatitis A virus infection, acute liver inflammation is found to be caused by hepatocyte apoptosis as an intrinsic response to infection. Science
13 September: A broadly neutralising antibody, which binds to the haemagglutinin stem and recognises most influenza A strains, is shown to be produced by human memory B cells. Nat Commun
2 September: Modelling suggests that the recently approved Sanofi-Pasteur vaccine may increase the frequency of severe dengue symptoms where viral transmission is low. Science
The 2009 flu pandemic was an influenza pandemic first recognised in Mexico City in March 2009 and declared over in August 2010. It involved a novel strain of H1N1 influenza virus with genes from five different viruses, which resulted when a previous triple reassortment of avian, swine and human influenza viruses further combined with Eurasian swine influenza viruses, leading to the term "swine flu" being used for the pandemic. It was the second pandemic to involve an H1N1 strain, the first being the 1918 "Spanish flu" pandemic.
The global infection rate was 11–21%. This pandemic strain was less lethal than previous ones, killing about 0.01–0.03% of those infected, compared with 2–3% for Spanish flu. Estimates of global fatalities range from 284,500 to 579,000, mainly in Africa and Southeast Asia – not much above the normal seasonal influenza fatalities of 250,000–500,000 – leading to claims that the World Health Organization had exaggerated the danger.
||...the variety of genes on the planet in viruses exceeds, or is likely to exceed, that in all of the rest of life combined.
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?
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
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.
A selection of recent articles of interest include: