Read more about the signs and symptoms of the most common STDs, how they spread, and how they're diagnosed and treated: Chlamydia. Genital Herpes. Genital Warts (HPV) Gonorrhea. HIV and AIDS. Pelvic Inflammatory Disease. Pubic Lice. Syphilis. Context: Chlamydia is the most common STD among teens. genital herpes. Definition: Also known as herpes simplex virus 2, a viral STD that causes sores on the genitals. genital warts. Definition: STD caused by human papillomavirus that causes warts on the genitals. gonorrhea. HIV/AIDS. STD. syphilis.