Influenza, also known as the flu, is a contagious disease that is caused by influenza viruses. Influenza viruses infect the respiratory tract (nose, throat, and lungs) in humans. The flu is different from a cold, mainly because the symptoms and complications are more severe. Influenza usually comes on suddenly and may include these symptoms: fever, headache, malaise (a feeling of being ill and without energy that can be extreme), cough, sore throat, nasal congestion and body aches. A lot of the illness and death caused by the flu can be prevented by a yearly flu vaccine. People in high-risk groups and people who are in close contact with those at high risk should get a flu vaccine every year as recommended by the Advisory Committee on Immunization Practices (ACIP). A flu vaccine can be given to anyone who wants to avoid the flu (persons over 6 months of age). Persons who provide important community services (such as police, fire department personnel, emergency medical services) should consider getting a flu vaccine so that those services are not disrupted during a flu outbreak.