Symptoms

The symptoms of influenza (flu) appear suddenly and often include:

• Fever of 100°F (37.8°C) to 104°F (40°C), which can reach 106°F (41°C) when symptoms first develop. Fever is usually continuous, but it may come and go. Fever may be lower in older adults than in children and younger adults. When fever is high, other symptoms usually are more severe.
• Body aches and muscle pain (often severe), commonly in the back, arms, or legs.
• Headache.
• Pain when you move your eyes.
• Fatigue, a general feeling of sickness (malaise), and loss of appetite.
• A dry cough, runny nose, and dry or sore throat. You may not notice these during the first few days of the illness when other symptoms are more severe. As your fever goes away, these symptoms may become more evident.

Influenza usually does not cause symptoms in the stomach or intestines, such as vomiting and diarrhea.

Other conditions have symptoms similar to the flu, such as the common cold, bacterial infections, and infectious mononucleosis.

What Happens

Influenza (flu) usually comes on suddenly. In many cases people can pinpoint the hour when symptoms started. Symptoms develop 1 to 4 days after you are infected, and they include:

• Fever, which lasts for about 3 days. Fever is usually slightly lower on the 2nd and 3rd days but may last up to 8 days.
• Cough, runny nose, and sore throat, which become more noticeable as fever and other symptoms decrease. These symptoms usually last 3 to 4 days after the fever goes down. A dry, hacking cough may linger for up to 10 days after other symptoms are gone.

Complete recovery may take 1 to 2 weeks or longer. Fatigue and weakness can last for several weeks.

Complications of influenza may develop in anyone, but they are much more likely in older adults and people who have other health problems, especially heart and lung diseases.