In the United States, 72% of all deaths among youth and young adults 5-24 years of age result from only four causes: motor vehicle crashes (28% of all deaths in this age group), other unintentional injuries (11%), homicide (21%), and suicide (12%). Substantial morbidity and social problems also result from the approximately 1 million pregnancies that occur each year among adolescents and the more than 10 million cases of sexually transmitted diseases (STDs) that occur each year among persons 15-29 years of age.