Influenza, commonly called the flu, is a contagious respiratory infection caused by influenza viruses. It is not the same as a common cold.
Common symptoms
Flu often starts suddenly and can cause fever, chills, cough, sore throat, body aches, headache, fatigue, and sometimes vomiting or diarrhea, especially in children.
Why it matters
Flu can lead to pneumonia, worsen asthma or COPD, and cause serious illness in older adults, young children, pregnant people, and people with certain medical conditions.
Flu, COVID-19, and RSV can look similar, so testing may be needed when the result affects care decisions.