While most children and healthy adults recover from the flu without suffering any long-term effects, children less than five years and people older than sixty-five are much more likely to experience serious illness and deaths. Common complications from the flu include pneumonia, ear infection, muscle breakdown, and dehydration. Life-threatening problems, including swelling of the brain and coma, can occur, but they are less common.

During a flu pandemic, the circulating flu virus is typically much more aggressive. In some situations, more than half will die from the infection.