Influenza (also known as the flu) is a contagious respiratory illness caused by flu viruses. It can cause mild to severe illness, and at times can lead to death. The flu is different from a cold. The flu usually comes on suddenly and may include these symptoms:
Fever (usually high)
Runny or stuffy nose
Stomach symptoms, such as nausea, vomiting, and diarrhea, also can occur but are more common in children than adults
These symptoms are usually referred to as "flu-like symptoms."
Anyone Can Get the Flu, But the Disease Is More Severe for Some People
Most people who get influenza will recover in a few days to less than 2 weeks, but some people will develop life-threatening complications (such as pneumonia) as a result of the flu. Millions of people in the United States-about 5% to 20% of U.S. residents-will get influenza each year. An average of about 36,000 people per year in the United States die from influenza-related causes, and more than 200,000 have to be admitted to the hospital as a result of influenza-related causes. Anyone can get the flu (even healthy people), and serious problems from influenza can happen at any age. People age 65 years and older, people of any age with chronic medical conditions (such as asthma, diabetes, or heart disease), pregnant women, and young children are more likely to get complications from influenza. Pneumonia, bronchitis, and sinus and ear infections are three examples of complications from flu. The flu can make chronic health problems worse. For example, people with asthma may experience asthma attacks while they have the flu, and people with chronic congestive heart failure may have worsening of this condition that is triggered by the flu. More information about the new H1N1 virus is available here.
How To Know if You Have the Flu
Your respiratory illness might be the flu if you have sudden onset of body aches, high fever, and respiratory symptoms, and your illness occurs during the usual flu season in the Northern Hemisphere. However, during this time, other respiratory illnesses can cause similar symptoms to the flu. In addition, influenza can also occur outside of the typical flu season. It is impossible to tell for sure if you have the flu based on symptoms alone. Doctors can perform tests to see if you have the flu if you are in the first few days of your illness.