Overview of Sexually Transmitted Infections By Sheldon R Morris, MD, MPH, University of California San Diego Reviewed / Revised Jan 2023 View Patient Education Symptoms and Signs Diagnosis Treatment Prevention More Information Topic Resources Differentiating Common Sexually... Sexually transmitted infection (STI) refers to infection with a pathogen that is transmitted through blood, semen, vaginal fluids, or other body fluids during oral, anal, or genital sex with an infected partner.