In the United States, 70.8% of all deaths among youth and young adults aged 10-24 years result from only four causes: motor-vehicle crashes (32.3%), other unintentional injuries (11.7%), homicide (15.1%), and suicide (11.7%). Substantial morbidity and social problems also result from the approximately 870,000 pregnancies that occur each year among females aged 15-19 years and the estimated 3 million cases of sexually transmitted diseases (STDs) that occur each year among persons aged 10-19 years.