Each year, an estimated 56,300 people in the U.S. become newly infected with HIV, the virus that causes AIDS—in part because about one-quarter of the more than 1.1 million Americans believed to be living with HIV don’t know they are infected, according to the Centers for Disease Control and Prevention. HIV is transmitted primarily through sexual intercourse or contact with infected blood, semen, or cervical and vaginal fluids; injection-drug use; and perinatally from infected mothers to their infants.