Caused by the influenza virus, the flu is an acute infection of the upper respiratory tract that has a more rapid onset and more severe symptoms than the common cold. The virus implants into the membranes that line the nose and throat and causes them to over-produce mucus and become swollen and inflamed as the body mounts an immune defense. This immune response results in nasal and respiratory congestion, coughing, sore throat and sneezing. Bodyaches, fever, chills, headache and fatigue normally accompany a flu infection. Herbs help to boost immune function and strengthen the body's innate defenses, speed healing time and provide symptomatic relief.