Influenza, often called the flu, is a viral infection that affects your lungs and airways.

Flu symptoms are a little bit like the common cold but are much more severe.

The flu spreads easily from person to person, sometimes causing epidemics (when many people get sick within a very short time).

The flu causes chills, fever, headache, sore throat, and cough. Most people recover, but flu makes some people very sick, and some die from pneumonia (a bad lung infection).

Doctors will have you rest and drink plenty of fluids.

What causes influenza?

The virus spreads through the air and droplets that an infected person spreads through a cough or sneeze and through touching things that infected people touched after wiping or blowing their nose. Flu season in the United States lasts from November through March. Each year, millions of people get the flu and thousands die. The flu virus changes a little bit from year to year, and some changes make the virus more deadly.

What are symptoms of the flu?

Early symptoms of flu include chills; fever up to 103°F or 39.4°C; sick, weak, tired feeling; muscle aches, especially in your back and legs; severe headache; runny nose; and scratchy sore throat. Later symptoms include severe cough that brings up phlegm or mucus, feeling sick to your stomach, and throwing up. Most symptoms go away within a week, but the cough may last for weeks. Children, pregnant women, older people, people with other diseases (such as diabetes, heart disease, and lung disease), and people with a weak immune system may have more severe symptoms. They may also get pneumonia.

How can doctors tell if I have the flu?

Doctors can usually tell if you have the flu based on your symptoms, especially if many people in your area have it too. To know for sure, doctors may test your blood or fluid from your nose or throat for the flu virus. If doctors think you may have pneumonia, they'll do a chest x-ray and pulse oximetry (a test that uses a sensor placed on your finger to see how much oxygen is in your blood).

How do doctors treat the flu?

Doctors will tell you to get lots of rest and drink plenty of fluids, especially until your fever goes away.

How can I prevent the flu?

Take these measures: