Flu signs and symptoms usually come on suddenly and can include fever/chills, cough, sore throat, runny nose, muscle aches, and fatigue (tiredness).
Pneumonia, bronchitis, sinus infections, and ear infections are examples of flu-related complications.
Flu can cause some medical conditions, like heart disease, cancer, or diabetes worse.
Yearly influenza epidemics can affect all populations, but children younger than the age of two, adults older than 65, and people with chronic medical conditions or weakened immune systems bear the highest risk of complications. The annual rate of flu cases is estimated at 5–10% in adults and 20–30% in children.