Influenza, also known as the flu, is a contagious disease that is caused by the influenza virus. It attacks the respiratory tract in humans (nose, throat, and lungs). The flu is different from a cold. Influenza usually comes on suddenly and may include these symptoms: fever, headache, tiredness (can be extreme), dry cough, sore throat, nasal congestion and body aches. These symptoms are usually referred to as "flu-like symptoms.
Fact: On average, more than 226,000 people are hospitalized from flu complications, including 20,000 children; about 36,000 people die from influenza each year. Being vaccinated provides the best protection available from the influenza virus– even when the vaccine does not closely match circulating flu strains. A vaccination may lessen influenza illness severity and is important for persons at high risk for serious flu-related complications and for close contacts of high-risk individuals.
Ref: http://www.cdc.gov/flu/about/qa/flushot.htm and http://www.cdc.gov/flu/about/qa/disease.htm.