Researchers from the CDC have stated that teenagers often do not understand the risks associated with sexual activity. "Research suggests that adolescents perceive fewer health-related risks for oral sex compared with vaginal intercourse. However, young people, particularly those who have oral sex before their first vaginal intercourse, may still be placing themselves at risk of STIs or HIV before they are ever at risk of pregnancy." "Several studies have documented that oral sex can transmit certain STIs, including chlamydia, genital herpes, gonorrhea and syphilis. Teenagers and young adults engaging in sexual activity are at increased risk of STIs or HIV."