Each winter, millions of people suffer from the flu. Flu - the short name for influenza - is caused by viruses. Viruses are very small germs. Some viruses can spread easily from one person to another. They cause illnesses or infections like the flu.
For some people, flu is a mild illness. For other people, especially the very young, the elderly, and those who have health problems like diabetes or heart disease, the flu can be very serious.
Most people who get the flu feel much better in a week or two. But, some people can get very sick. For example, because your body is busy fighting off the flu, you might pick up a second infection. Other people, such as those listed above, are at great risk of these infections, such as pneumonia.
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