Sexually transmitted diseases are a definition used for diseases that are transmitted from one person to another through sexual contact. This group of diseases is also called sexually transmitted infections. Bacteria, viruses, or fungi that cause STDs can be passed from one person to another through blood, semen, vaginal fluid, or other body fluids.

Sexually transmitted diseases can spread between people without sexual contact. For example, these infections can be passed from mother to baby during pregnancy or birth. Blood transfusion (transfusion), especially in drug users, use of shared needles are among the ways of transmission of sexually transmitted diseases.