Influenza (commonly called "the flu") is a contagious respiratory illness caused by influenza viruses. Infection with influenza viruses can cause severe illness and life-threatening complications. About 10% to 20% of Americans get influenza each year, about 36,000 Americans die from it each year, and many more are hospitalized

Symptoms of flu include fever (usually high - 100 degrees F or more), headache, extreme tiredness, dry cough, sore throat, runny or stuffy nose, and muscle aches. Illness usually lasts for 2 to 7 days, but complete recovery may take longer. Most people recover completely, but influenza can sometimes worsen other health problems, which may lead to death.

"Stomach Flu"
Symptoms of the stomach and intestines, like nausea, vomiting, and diarrhea, are rare in adults but are sometimes seen in children. Some people call diarrhea, nausea, or vomiting that lasts for a day or two 'stomach flu', but that illness is very different from true influenza.

Influenza is hard to diagnose because there are several other diseases that resemble it. Tests are available that can tell if you have influenza if you are tested within the first 2 or 3 days after your symptoms begin. In addition, a doctor's examination may be needed to determine whether a person has another infection that is a complication from influenza.

Some of the complications caused by influenza include bacterial pneumonia, dehydration, and worsening of chronic medical conditions such as congestive heart failure, asthma, or diabetes. Children may get sinus problems and ear infections. Complications can be severe and may lead to death.

Certain people are at increased risk for influenza-related complications and severe disease. This group includes people age 50 years and older and people of any age with chronic medical conditions. Pregnant women and children between 6 months and 59 months of age are also at increased risk from flu complications.