Infectious parotitis See mumps

influenza (flu) A contagious respiratory infection that often occurs in epidemics. The disease is most dangerous not in itself, but because it can lead to PNEUMONIA, especially among older people and those with impaired immune systems. When complicated by pneumonia, the flu is the sixth most common cause of death in the United States, killing 20,000 Americans.

Influenza occurs most often in the winter months. Although illnesses like the flu may occur in summer, these are usually caused by other viruses. Every year as winter begins, the flu spreads across the globe; in the United States, up to 50 million people will be infected. The flu is responsible for about three days of lost work per adult, and field studies indicate the attack rate ranges from a low of about 10 percent in people over age 65 to a high of 36 percent in children from ages 1 to 18. At the peak of a typical epidemic, up to 22 percent of all physician visits are for flulike symptoms.

While more than 90 percent of flu-related deaths occur among the elderly, children under age 5 and women in the third trimester of pregnancy are also at higher risk for complications. The word flu is a slang term that applies to a variety of types of viral GASTROENTERITIS (stomach or intestinal flu), which are not really related to the true respiratory influenza caused by influenza virus.

History Influenza is as old as human history; the mysterious 430 B.C. deadly plague in Athens may have been caused in part by deadly flu viruses. Historians also suspect that the mighty army of Charlemagne was destroyed by a flu epidemic of A.D. 876. The first true recorded flu pandemic occurred in the 16th century. In 1647, the flu was causing havoc in North America up through New England, where it was known variously as "jolly rant," "grippe," and "the new acquaintance." Its current name was bestowed after the 1732 epidemic in the American colonies, when English doctor John Huxham linked the disease with an old Italian folk word that linked colds, cough, and fever to the "influ ence" of the stars. About every 20 to 50 years, another pandemic (worldwide epidemic) sweeps across Earth, with yearly local epidemics in between. Major pandemics occurred in 1627, 1729, 1788, 1830, 1847, 1872, 1890,1918,1957, and 1968.

Several times in this century, influenza has appeared as a much more serious pandemic; these major episodes occur when the flu virus undergoes an "antigenic shift" in which one flu subtype is replaced by a different strain for which the population has not developed antibodies. Therefore, everyone is extremely susceptible to infection.

Still, because there were far more deadly diseases to worry about, even in the beginning of this century the flu did not attract much medical attention until the great Spanish flu pandemic of 1918, the worst of the pandemics, killing 10 million more people than did World War I. In the United States alone, 550,000 people died. The Spanish flu of 1918 left about 21 million dead out of about 1 billion cases before it vanished; scientists still don't know where it went and worry that another outbreak could occur. Its name was particularly insulting to Spaniards, since this particular pandemic appeared to have actually begun in the United States.

The 1918 pandemic was not restricted to the big cities, although more than 20,000 New Yorkers were killed in the fall of 1918. Whole Inuit villages in Alaska were decimated, and Samoa lost 20 percent of its people.

Since World War II, vaccines have helped cut the death rate, which was very low in the 1957 pandemic of Asian flu, and in the 1968 pandemic. In 1976, an outbreak of swine flu in Fort Dix, New Jersey, set off alarms throughout the United States, since it was swine flu that was believed to have caused the mass mortality in 1918, although no one knows for sure. Then-president Gerald Ford signed a law providing $135 million for a vaccine campaign that reached about a quarter of the population. The United States and Canada set up a crash mass vaccination program; however, when some people who had been vaccinated developed Guillain-Barre syndrome (a rare type of temporary paralysis) the United States canceled its program. The dreaded pandemic never developed, and the U.S. government eventually paid about $93 million in damages to Guillain-Barre victims.

Cause The influenza virus was first named Hemophilus bacterium influenzae, but this was never proved conclusively. In the same year, W. Smith, F. W. Andrewes, and P. P. Laidlaw proved the cause of influenza was a virus that was experimentally transmissible to ferrets. This virus is now identified as influenza A. Influenza B virus was discovered in 1940, and influenza C was identified nine years after that. At the moment, the virus exists as one of these three types. However, types A and B mutate quickly; several strains of each of these types now exist. The three basic flu types have variants that are designated according to where they first strike, such as New Jersey (A), Bangkok (A), and so on. The 1957 and 1968 Asian flu pandemics were caused by strains of type A.

The highly contagious infection is spread by direct contact or via droplets and dust in the air over short distances from patients who are coughing or sneezing. The virus can also survive for hours in dried mucous, so dirty tissues should be carefully disposed of. Anyone can catch the flu, regardless of age, sex, or race, but certain groups are more likely to develop complications of the disease. Deaths occur primarily among those over age 65, or those with certain chronic diseases.

The contagious period varies, but it probably starts the day before symptoms appear and extends for about a week.

An attack will confer immunity to the specific strain only. Because the viruses that cause flu are always changing (mutating), people who have been infected or who have gotten a flu shot in other years may become infected with a new strain.

Symptoms About one or two days after exposure, symptoms of flu develop suddenly, with fever, headache, and body aches. Intestinal symptoms are uncommon. The throat is sore, dry, and red. A cough appears on the second or third day, followed by a drop in fever with drenching sweats by the third to fifth day. As the fever drops, the patient becomes highly susceptible to secondary bacterial invasion. Fatigue and depression may last for weeks afterward.

Although most people are sick for only a few days, some people have a much more serious illness (such as pneumonia) and may need to be hospitalized.

Diagnosis There is no easy way to diagnose influenza. While the virus can be isolated from the throat, and antibodies can be found in the blood, these tests are expensive and slow. Diagnosis is usually made on the basis of the symptoms and the occurrence of other cases in the area.

Treatment There is no specific cure for the flu. No known antibiotic has any effect on any type or strain of virus, although antibiotics may be used to treat a secondary bacterial pneumonia. Treatment is usually aimed at reducing fever and relieving symptoms. Rest and liquids are usually adequate.

In addition to the vaccine, two oral prescription drugs (amantadine and rimantadine) may prevent or reduce the severity of influenza A but are not effective against type B (which accounts for 30 percent of flu cases). Rimantadine (Flumadine) or amantadine (Symmetrel) are designed to shorten illness by preventing the flu virus from reproducing. They work best if taken as soon as the flu strikes. Rimantadine is a closely related product that produces fewer side effects and can be used safely by adults; it is being tested for safety in children.

Side effects, including nausea, insomnia, and impaired concentration affect about 1 in every 30 patients. Amantadine causes similar side effects in 1 of every 10 patients; it's cheaper than rimantadine when bought as a generic. An antiviral may help even if you've been sick for several days, but once you start to get better they probably won't be very effective.

Prevention Routine vaccination against the flu is the most important way to control the disease. Vaccines are available through personal physicians or the local health department. Research has shown that even in years when new strains emerge, people in high-risk groups who get yearly flu shots tend to have milder illnesses and are less likely to be hospitalized with complications due to influenza A.

The first practical vaccination against the flu was developed in 1943 with killed viruses of both types A and B. Because the influenza viruses are constantly evolving, the vaccines must continually be updated; as new strains of the virus appear, they are included in the vaccine.

Each year, scientists at the U.S. Centers for Disease Control and the World Health Organization make an educated guess about which kind of flu will predominate during the next winter season. These two groups maintain a global network that collects data required to select strains for the coming flu season's vaccine, and monitor the occurrence of especially severe epidemics. A similar process is undertaken in Europe by the WHO and various national authorities there.

In this country, for example, when developing the flu vaccine for the winter of 1994-95, CDC scientists studied flu viruses from 1,500 samples around the world. They chose in March 1994 to include vaccine made from three strains, A-Texas, B-Panama, and A-Shangdong; four drug companies then manufactured 70 million doses of vaccine.

The U.S. Public Health Service recommends annual vaccination only for those most likely to develop complications from influenza. This includes anyone over age 65 or anyone with a chronic debilitating disease such as heart disease, chronic respiratory dis ease (such as asthma, bronchitis, or emphysema), or a chronic metabolic disorder (such as diabetes). Other people at high risk are residents of nursing homes and other institutions housing patients of any age with serious long-term health problems, people with kidney disease, cystic fibrosis, anemia, asthma, cancer, or immunological disorders. In addition, household contacts of high-risk people, and health care workers who care for those at high risk, should be vaccinated. The shots are 70 to 80 percent effective in preventing flu.

Immunity to the strains of flu in a particular vaccine occurs within about a week or two of getting the shot. Because the vaccine is made from killed virus, you can't get the flu from a shot.

Because the viruses that cause flu are always changing (mutating), people who have been infected or who have gotten a flu shot in other years may become infected with a new strain. Because of this and because immunity from an earlier flu shot can decrease the next year, people in high-risk groups should be vaccinated each year.

People with severe allergies to eggs should avoid the shot.

In the fall of 1997, a limited recall of flu vaccine was necessary for the first time since flu vaccines had been distributed because certain batches did not fully protect against the disease. Those who had been immunized with the weakend vaccine were asked to be revaccinated to protect against disease.

New research California scientists are preparing to conduct human tests of an aspirin-sized pill containing an ingredient that rids lab animals of flu symptoms within a day. The drug works by disrupting an enzyme found on the surface of major strains of flu virus. Scientists at Gilead Sciences in Foster City were expected to try the drug on Europeans in 1997 to see if it has the same effect on humans as it did on mice and ferrets. While ferrets contract the same type of flu that humans get, scientists cautioned that often,

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