性行為
Sexual intercourse can also be a disease vector. There are 19 million new cases of sexually transmitted diseases (STD) every year in the U.S., and worldwide there are over 340 million STD infections each year. More than half of these occur in adolescents and young adults aged 15–24 years. At least one in four U.S. teenage girls has a sexually transmitted disease. In the U.S., about 30% of 15- to 17-year-olds have had sexual intercourse, but only about 80% of 15- to 19-year-olds report using condoms for their first sexual intercourse. In one study, more than 75% of young women age 18–25 years felt they were at low risk of acquiring an STD.