Entitled: "Le Gros Lot ou les ?trennes imprevues" ("The Jackpot or unforeseen Strennas"), two men suffering from venereal disease stand at the door of a physician. Etching by Achille L. Martinet, 19th century. Sexually transmitted diseases (STD), also referred to as sexually transmitted infections (STI) and venereal diseases (VD), are illnesses that have a significant probability of transmission between humans by means of sexual behavior, including vaginal intercourse, anal sex and oral sex. Sexually transmitted infections have been well known for hundreds of years, and venereology is the branch of medicine that studies these diseases.