Within 2 years of having sex for the first time, half of teenage girls may be infected with at least one of three common sexually transmitted diseases (STDs), according to results of a new study. Often, those girls are infected by the age of 15. Researchers followed 386 urban adolescent girls aged 14 to 17 for up to 8 years. Within 2 years of becoming sexually active, half of the girls were infected with at least one of three common sexually transmitted organisms: Chlamydia trachomatis, Neisseria gonorrhoeae, or Trichomonas vaginalis — the organisms that cause chlamydia, gonorrhea and trichomoniasis, respectively....