Viruses are small
infectious agents that can replicate only inside the living
cells of an organism. Viruses infect all forms of life, including
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
RNA, wrapped in a
protein coat called the
capsid; some viruses also have an outer
envelope. The capsid can take simple
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
The origins of viruses are unclear: some may have
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".
Influenza, or flu, is an
infectious disease caused by some
orthomyxoviruses, that affects
birds and some
mammals including humans, horses and pigs. Influenza is predominantly
transmitted through the air by coughs or sneezes, creating
aerosols containing the virus. It can also be transmitted by contact with bird droppings or
nasal secretions, or by touching contaminated surfaces. As the virus can be inactivated by soap, frequent hand washing reduces the risk of infection. Around a third of cases show no symptoms. The most common symptoms include
fatigue. Influenza is occasionally associated with
vomiting, particularly in children.
Pneumonia is a rare complication which can be life-threatening.
Influenza spreads around the world in
seasonal epidemics, resulting in about 3–5 million cases of severe illness annually, and about 250,000–500,000 deaths, mainly in the young, the old and those with other health problems. Annual
influenza vaccinations are recommended for those at high risk. Sporadic
influenza pandemics have been recorded since at least the 16th century. The
Spanish flu pandemic of 1918–20 is estimated to have killed 50–100 million people.
In the news
Map showing the prevalence of SARS-CoV-2 cases; black: highest prevalence; dark red to pink: decreasing prevalence; grey: no recorded cases or no data
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.
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.
14 February: Seven cases of
Ebola virus disease are reported in
7 February: A case of Ebola virus disease is detected in
North Kivu Province of the
Democratic Republic of the Congo.
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.
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.
18 November: The outbreak of
Ebola virus disease in
Democratic Republic of the Congo, which started in June, has been declared over; a total of 130 cases were recorded, with 55 deaths.
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
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. Reversion of live vaccine strains to
virulence has resulted in occasional cases of vaccine-associated
Two of the three strains of wild-type poliovirus have been eradicated. Annual reported cases of wild poliovirus infection fell to a low of 22 in 2017, but had risen to 176 in 2019. As of 2020, the wild virus remains
endemic only in Afghanistan and Pakistan, but vaccine-derived poliovirus is circulating in several countries. 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.
2001 foot-and-mouth outbreak included 2,000 cases of
the disease in cattle and sheep across the UK. The source was a
Northumberland farm where pigs had been fed infected meat that had not been adequately sterilised. The initial cases were reported in February. The disease was concentrated in western and northern England, southern Scotland and Wales, with
Cumbria being the worst-affected area. A small outbreak occurred in the Netherlands, and there were a few cases elsewhere in Europe.
The UK outbreak was controlled by the beginning of October. Control measures included stopping livestock movement and slaughtering over 6 million cows and sheep. Public access to farmland and
moorland was also restricted (pictured), greatly reducing
tourism in affected areas, particularly in the
Lake District. Vaccination was used in the Netherlands, but not in the UK due to concerns that vaccinated livestock could not be exported. The outbreak cost an estimated £8 billion in the UK.
||...the variety of genes on the planet in viruses exceeds, or is likely to exceed, that in all of the rest of life combined.
Noroviruses are a
genus of non-
RNA viruses in the family
genome is approximately 7500
nucleotides long. Known noroviruses fall into five different genogroups (GI–GV); three groups infect humans, the other two mice, and cattle and other
bovines. All are considered strains of a single species,
Noroviruses are extremely contagious, with fewer than 20 virus particles being infectious. They are transmitted directly from person to person and indirectly via contaminated water and food. After infection, the virus replicates in the
small intestine, causing acute
gastroenteritis, which develops 12–48 hours after exposure and lasts for 24–72 hours. The characteristic symptoms include
diarrhoea and abdominal pain. Infection is usually
self-limiting and rarely severe. Noroviruses cause 18% of acute gastroenteritis episodes in humans, with around 685 million cases and 200,000 deaths every year, mainly in very young, elderly or immunosuppressed people. No
vaccine is available. Hand washing with soap and water is effective in reducing transmission.
Did you know?
Sir Frank Macfarlane Burnet (3 September 1899 – 31 August 1985) was an Australian
immunologist. His early virological studies were on
bacteriophages, including the pioneering observation that bacteriophages could exist as a stable non-infectious form that multiplies with the bacterial host, later termed the
With the outbreak of World War II, Burnet's focus moved to
influenza. Although his efforts to develop a live
vaccine proved unsuccessful, he developed
assays for the isolation, culture and detection of
influenza virus, including
haemagglutination assays. Modern methods for producing
influenza vaccines are still based on his work improving virus-growing processes in hen's eggs. He also researched influenza virus genetics, examining the genetic control of
virulence and demonstrating, several years before influenza virus was shown to have a segmented genome, that the virus
recombined at high frequency.
In this month
human papillomavirus (HPV) vaccines, including
Gardasil, have been approved to protect against infections with particular types of
HPV, associated with
cervical and other cancers. All vaccines protect against the high-risk HPV types 16 and 18. Gardasil is a quadrivalent vaccine that additionally protects against low-risk HPV-6 and -11, which are associated with most cases of
genital warts. A second-generation nine-valent Gardasil vaccine protects against five additional high-risk HPV types. It is estimated that the vaccines may prevent 70% of cervical cancer, 80% of
anal cancer, 60% of
vaginal cancer, 40% of
vulvar cancer and possibly some
oropharyngeal cancers. Protection lasts for at least 8–9 years. Some advocate giving Gardasil to men and boys with the primary aim of protecting their female sexual partners; others consider vaccinating only women and girls to be more cost effective. The licensed vaccines are
subunit vaccines, containing only the L1
capsid protein of the virus, which self-assembles into
virus-like particles. They are not effective in people already infected with HPV. Research is ongoing into therapeutic HPV vaccines including the viral
oncoproteins, E6 and E7, but none has yet been licensed.