Around 5 to 20 percent of the United States population gets the flu annually, with over 200,000 infections resulting in hospitalizations, according to the Centers for Disease Control and Prevention. Influenza is caused by viruses that infect the respiratory system.
There are three types of influenza, including type A, B and C. Influenza A viruses, such as H1N1 and H3N2, are the most common among humans, notes the World Health Organization. Symptoms of the flu include fever, cough, headache, sore throat and joint pain. Influenza can cause severe illness or death in high-risk people such as children under the age of 2, pregnant women and people with certain medical conditions.