If you have the flu, you'll have lots of company. Each year from November to April, all across the United States, as many as 60 million people come down with the flu. Although children get the flu most often, people in every age group - including teens - can catch it

What Is the Flu?
Flu is the common name for influenza (pronounced: in-floo-en-za), a virus that attacks the respiratory system.

Usually when you're sick with a virus, your body builds up a defense system by making antibodies against it, so you don't get that particular virus strain again. Unfortunately, flu viruses mutate (change) each year, so you aren't protected from getting the flu forever.

Some years the change in the flu virus is slight. So even if you get the flu, it's a mild case because the antibodies from having the flu before give you partial protection.

But every 10 years or so the flu virus undergoes a major change and many people get severe cases. These large-scale outbreaks are called epidemics. If they spread worldwide, they're called pandemics.