Every winter, like clockwork, the flu returns. It inf­ects millions of ­us -- about 5 to 20 percent of the U.S. population alone, according to the Centers for Disease Control and Prevention (CDC) [source: CDC]. Health Canada estimates that 10 to 25 percent of Canadians get the flu each year [source: Health Canada]. It leaves us sniffling, sneezing, coughing, achy and generally feeling miserable for anywhere from a few days to a few weeks.

Although most of us think of the flu as a mild annoyance that we have to deal with each winter, it can actually be a very dangerous disease. The CDC estimates that in the United States alone, more than 200,000 people are hospitalized­ with the flu or with flu-related complications each year, and more than 36,000 people die from it [source: CDC]. Around the world, the flu kills between a quarter of a million to half a million people a year [source: WHO].

­In this article, we'll find out how people get the flu, what the symptoms are and how to protect yourself.