Adolescents and young adults (15-24) are the age groups at the greatest risk for acquiring an STD. Approximately two-thirds of all people who acquire STDs are under 25. The Centers for Disease Control states that adolescents and young adults are at greater risk for many reasons, including: They may have less immunity than adults They may be more likely to engage in unprotected intercourse They may select partners at higher risk Age at initiation of sexual activity has decreased while age at first marriage has increased, resulting in more nonmarital sexual experience They may be more likely to have multiple sex partners According to former U.S. Surgeon General, C. Everett Koop, "when you have sex with someone, you are having sex with everyone they have had sex with for the last 10 years, and everyone they have had sex with for the last 10 years."