Syphilis or, more properly, venereal syphilis is a chronic communicable disease, which, until the acquired immunodeficiency syndrome emerged in the early 1980s, was the most serious and dreaded of the so-called sexually transmitted diseases (STD) – formerly, venereal diseases (VD). Caused by Treponema pallidum subspecies pallidum, a spirochetal bacterium, the only known natural host of which is the human being, venereal syphilis is thus one of the human treponematoses – along with pinta, yaws, and endemic syphilis. Although predominantly transferred by sexual contact, T. pallidum is also capable of being transmitted from an infected mother to her fetus across the placenta at any stage of pregnancy (congential syphilis).

Syphilis develops naturally through three clinical stages (primary, secondary, and tertiary or late), each separated by a subclinical period. Of the subclinical periods, the one between the secondary and tertiary stages (latent syphilis) is the most pronounced. Clinical manifestations of syphilis are extremely protean, and capable, at the tertiary stage, of affecting any system of the human body.

Syphilis took its name from Girolamo Fracastoro’s well-known poem, Syphilis, sive morbus gallicus (1530), in which the Italian humanist-physician invented this phrase to name the disease then known all over Europe as morbus gallicus. However, the term syphilis did not become widely used until the late eighteenth century, and that usage was vague and applied to many other symptoms besides those of venereal syphilis until the development of the germ theory in the late nineteenth and early twentieth centuries.