Flu is a common viral infection that can be deadly, especially in high-risk groups.

Signs and Symptoms

Influenza, commonly known as the "flu," is an extremely contagious respiratory illness caused by influenza A or B viruses. Flu appears most frequently in winter and early spring. The flu virus attacks the body by spreading through the upper and/or lower respiratory tract. Common signs and symptoms of the flu include: Fever over 100 F (38 C) Aching muscles, especially in your back, arms and legs Chills and sweats Headache Dry, persistent cough Fatigue and weakness Nasal congestion Sore throat