Influenza (flu) is a common, contagious respiratory disease that infects the nose, throat and lungs. Severeness of the illness can range from mild to very severe cases. Symptoms of the flu include fever, fatigue, vomiting, sore throat, and body aches. The flu can also lead to other illnesses, such as pneumonia, and can even lead to death. To help protect your body against the flu, doctors recommend getting the flu vaccine annually. Vaccination has been shown to help lower the risk of flu illnesses. It takes about two weeks after vaccination for the body to build the antibodies needed for protection against the flu.