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Influenza ("the flu") is a disease caused by the influenza virus. The disease is easily spread from person to person, typically by inhaling virus that has been expelled into the air by coughing or sneezing. The virus can also be spread by touch. For example, if someone touches a doorknob that has influenza viruses clinging to it and then touches their mouth, the virus can pass into their body and cause influenza.

The influenza virus infects the nose, throat, and lungs of people. In contrast to the common cold, which is caused by a different virus, the symptoms of the flu develop suddenly. These symptoms include fever, headache and body aches, tiredness, cough, sore throat, and stuffy nose.

Most people who contract the flu recover completely in a few weeks. However, in some people influenza can progress to pneumonia, which can be life threatening. Recovery from influenza does not protect someone from future bouts of the disease. This is because the influenza virus readily changes the expressions of its genetic material (i.e., it mutates readily). Thus, the influenza virus that the body's immune system responds to one season may be different from the virus that infects the body some months later.

Influenza is a common illness. For example, every year approximately 25 to 50 million Americans (about 10–20% of the population of the United States) contract influenza. Of these, about 20,000 people die of the infection, and 114,000 require hospitalization.

There are three types of influenza virus. All three are in the viral group called Orthomyxovirus. The three viral types are called influenza A, B, and C. Influenza A and B cause large numbers of cases of the flu almost every winter, when people are confined indoors and spread of the virus is easier.

Influenza A is further divided into two subtypes called hemagglutinin (H) and neuraminidase (N). H and N are two proteins that are found on the surface of the filament-like virus particles. They both protrude from the surface and appear as spikes when viruses are examined under high magnification. The protein spikes function to help the virus invade host cells.

Influenza viruses are resident in animals and birds including pigs, horses, seals, whales, a variety of wild birds, and ducks. The virus can spread from this reservoir to humans.

Influenza has been part of mankind for millennia. In the twentieth century, there were a number of large outbreaks. For example, in 1918–1919 the "Spanish flu" killed more than 500,000 people in the United States and up to 50 million people around the world. The influenza virus that caused this outbreak was very deadly. Concerns have been raised that the same virus could establish another epidemic.

In 1957–1958 the "Asian flu" caused 70,000 deaths in the United States. The same virus remains in circulation today. In 1968–1969, an outbreak of what was dubbed the "Hong Kong flu" killed approximately 34,000 Americans. In 1976, a small outbreak affected soldiers at a military base in Fort Dix, New Jersey. Experts predicted that the influenza, which was known as the "Swine flu," could spread throughout the United States. The subsequent public concern bordered on hysteria, and prompted a vaccination campaign in which 40 million Americans were vaccinated. The outbreak did not materialize.

Vaccination is not a guarantee that all types of influenza will be prevented. Rather, influenza is typically dealt with after it appears. Flu is treated with rest and fluids. Maintaining a high fluid intake is important, because fluids increase the flow of respiratory secretions that may prevent pneumonia. Antiviral medications such as amantadine and rimantadine may be prescribed for people who have initial symptoms of the flu and who are at high risk for complications. This medication does not prevent the illness, but reduces its duration and severity.

A flu vaccine is available that is formulated each year against the current type and strain of flu virus. The A transmission electron micrograph (TEM) of influenza viruses budding from the surface of an infected cell. CNRI/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.
virus is grown in chicken eggs, extracted, and then rendered noninfective by chemicals. The vaccine is also "updated" to the current viral strain by the addition of proteins that match the protein composition of the influenza virus type that is currently circulating in a population. The vaccine would be most effective in reducing attack rates if it was effective in preventing influenza in schoolchildren; however, in vaccine trials the vaccine has not been shown to be effective in flu prevention in this age group. In certain populations, particularly the elderly, the vaccine is effective in preventing serious complications of influenza and thus, lowers mortality.

Vaccine research is ongoing. One of the more exciting advances in flu vaccines involves research studies examining an influenza vaccine mist, which is sprayed into the nose. This is predicted to be an excellent route of administration, which will confer even stronger immunity against influenza. Because it uses a live virus, it encourages a strong immune response. Furthermore, it is thought to be a more acceptable immunization route for schoolchildren, who are an important reservoir of the influenza virus.

See also Aerosols; Cold, common.



Kolata, G.B. Flu: The Story of the Great Influenza Pandemic of 1918 and the Search for the Virus That Caused It. New York: Farrar Straus & Giroux, 1999.

Potter, C.W. Influenza London: Elsevier Health Sciences, 2002.


Centers for Disease Control and Prevention, 1600 Clinton Road, Atlanta, GA 30333 (404) 639–3311. July 29, 2002 [cited November 12, 2002] <http://www.cdc.gov/ncidod/diseases/flu/fluinfo.htm.

Brian Hoyle

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