The flu is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and at times can lead to death. The best way to prevent the flu is by getting a flu vaccination each year.
Every year in the United States, on average:

Some people, such as older people, young children, and people with certain health conditions, are at high risk for serious flu complications.

Symptoms of Flu

Symptoms of flu include:

Complications of Flu

Complications of flu can include bacterial pneumonia, ear infections, sinus infections, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes.
 

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How Flu Spreads

Flu viruses spread mainly from person to person through coughing or sneezing of people with influenza. Sometimes people may become infected by touching something with flu viruses on it and then touching their mouth or nose. Most healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 days after becoming sick. That means that you may be able to pass on the flu to someone else before you know you are sick, as well as while you are sick.

Preventing Seasonal Flu: Get Vaccinated

The single best way to prevent seasonal flu is to get a flu vaccination each year.