Signs and Symptoms
For most people, the seasonal flu is a moderate illness that causes symptoms such as fatigue, fever, chills, stuffy nose, sore throat, headache, muscles aches, weakness, a cough and, with some strains, even diarrhea and vomiting. These symptoms may also be seen with a variety of other conditions and seasonal viral infections.
Influenza can be severe and lead to complications such as viral pneumonia or secondary bacterial pneumonia (see Pneumonia) in the very young, the elderly, in those who are pregnant, and in those with underlying conditions such as asthma, lung disease, heart disease, diabetes, kidney disease, liver disease, and in those with compromised immune systems as with cancer or HIV/AIDS.
The incubation period for influenza is about two days. This is followed by several days of illness and then a resolution of symptoms. People shed virus and are infectious about a day before symptoms emerge and then for about five to seven days, or until about 24 hours after their fever ends.
Children and those with compromised immune systems may be infectious for longer periods of time.