The acronym STI stands for sexually transmitted infections while STD stands for sexually transmitted disease. STIs are infections that usually start in your sexual and reproductive organs. Some infections may also infect other parts of your body. STIs are very common: up to 75% of the sexually active population will get an STI of some kind in their lifetime. While some STIs may be painful, it is not unusual to have an STI without experiencing any symptoms for some time. Once symptoms do occur, an STI is considered an STD.