The flu usually starts suddenly and may include these symptoms:

• Fever (usually high)
• Headache
• Tiredness (can be extreme)
• Cough
• Sore throat
• Runny or stuffy nose
• Body aches
• Diarrhea and vomiting also can occur but are more common in children

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These symptoms are referred to as "flu-like symptoms." A lot of different illnesses, including the common cold, can have similar symptoms.

Know the Risk from the Flu

Some of the complications caused by the flu include bacterial pneumonia, dehydration and worsening of chronic medical conditions, such as congestive heart failure, asthma or diabetes. Children and adults may develop sinus problems and ear infections.