STD stands for sexually transmitted disease. Sexually transmitted diseases can also be called sexually transmitted infections (STIs). STDs are spread through sexual behavior or contact. Some STDs can be transmitted through skin-to-skin contact or body fluids such as blood, pre-cum, semen, vaginal fluids or breast milk. STDs generally infect the genital area (penis, scrotum, vulva and vaginal opening), anus or mouth, although they can spread to other parts of the body if left untreated.