Each year, according to the CDC, 5 to 20 percent of U.S. residents get the flu. For most people, the symptoms of flu begin abruptly and resolve completely in a week or two. However, some people take longer than others to recover.
Some people--particularly the elderly--get sicker than others for reasons that are not well defined. Dr. Raphael Dolin in "Harrison's Principles of Internal Medicine" reports that a "significant minority" of patients actually require several weeks to make a full recovery after a severe bout of influenza. These patients usually complain that weakness, increased need for sleep and inability to return to previous activity levels are the longest-lasting symptoms.
Flu-related complications such as pneumonia, ear infections, dehydration and encephalopathy--inflammation of the brain--significantly prolong recovery and may even result in hospitalization. Pneumonia, in particular, is the single most common complication of the flu. It may be caused by the influenza virus, by bacteria or both. With the exception of dehydration, most complications of flu present as worsening or new symptoms in a person who was previously starting to get better.
The CDC warns that exacerbation of chronic diseases such as asthma, heart disease and diabetes can also add to recovery time after the flu. Healthy people maintain reserves of heart and lung capacity that they can tap when needed. People with chronic diseases require medications or even more aggressive therapies such as supplemental oxygen. For these people, coping with the flu often means new medications, increased doses of existing medications and even admission to the hospital. It can take weeks or months to get back to normal, which is why the CDC recommends that people with medical problems get vaccinated against flu.