304. Flu season occurs annually in countries that have cold weather in the winter. Although the influenza virus is alive throughout the world in all four seasons, it seems to have its greatest impact on humans during the winter. Scientists are not sure why. They think it's because in the winter people are indoors and close together for longer periods of time. Also, the virus might be able to live longer and be stronger in colder weather. Finally, indoor air can dry up the mucous that helps protect people from the virus.

In the U.S., the virus is not something to sneeze at. It kills about 36,000 people annually (most of them elderly). It also causes almost 250,000 people to end up in a hospital bed annually.

Scientists develop a preventive flu shot for people to take as winter begins. This shot is not guaranteed to prevent flu; scientists can only guess at which virus might be the most dangerous in the upcoming season. The shot protects against only that particular virus. For those who do not get the shot, influenza spreads among them easily. It takes only a sneeze, a cough, or a touch. Victims frequently get ill from shaking hands, opening doors, or handling money.

Most infected people will have a fever, sore throat, and cough. But the symptoms are often mild and last for only a week. For many others, however, symptoms can be so severe and prolonged that victims end up in bed for weeks, or in the hospital, or in the cemetery.

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