Flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. It can cause mild to severe illness, and sometimes can lead to death. Symptoms of flu can include fever or a feverish feeling, cough, sore throat, runny or stuffy nose, muscle or body aches, headache, fatigue, vomiting, and diarrhea. Flu can be especially dangerous for young children, pregnant women, older people, and those with certain chronic medical conditions, such as asthma, diabetes, and neurological disorders. Flu can worsen chronic conditions and can lead to other complications, like pneumonia. An estimated 5 to 20 percent of the U.S. population becomes infected with the flu each year. About 200,000 of these flu sufferers are hospitalized each year, including 20,000 children under the age of 5. The Centers for Disease Control and Prevention (CDC) considers annual vaccination to be the single best way to prevent the flu. Parents and teachers are essential allies in efforts to increase awareness about the importance of flu vaccination, and encourage more people to protect themselves by getting vaccinated. This article discusses some facts about flu vaccination.