According to the Centers for Disease Control and Prevention, noroviruses are the most common cause of gastroenteritis in the United States.  

Every year, more than 21 million Americans become infected with norovirus, and about 800 of them die from it.  Due to the numerous strains and its ability to quickly mutate, it is difficult to develop a lasting immunity to norovirus.  As a result, you may become infected multiple times throughout your life.