Syphilis is caused by Treponema pallidum. About 3 weeks pass between infection and the appearance of an ulcer/wound on the skin and mucous membranes. A painless ulcer called an ulcus durum appears atn the site of infection. Ulcus durum may appear on the penis, on the external female genitals, in the anus/rectum, vagina/cervix or in themouth/on the lips, depending on the type of sexual intercourse causing the infection. About a week after the appearance of ulcus durum, the inguinal glands in the vicinity become swollen but not painful; 8 to 9 weeks after the infection, other glands swell and headache, fever and aching joints may appear. This is followed by a rash on the torso and limbs, frequently with reddish-brown spots on the palms and soles. Highly infectious road warts (Condylomata lata) may appear in the mucous membranes or skin of the genitals and anus. Four to five months after infection, hair loss may be experienced, usually in the form of totally bald spots of a few millimetres in diameter. After a year following infection, the changes on the skin and mucous membranes disappear and the disease progresses to the latent form, which is the most common form of late syphilis. After a few years, about half of patients experience destructive changes of the skin or internal organs, or vascular, heart or nervous system failure. Syphilis is treated by antibiotics. Since the prevalent view of both expert and lay publics is that syphilis has been eradicated, it is often overlooked and blood tests for confirmation are omitted.

In 2008 and 2009, a high increase of this dangerous infection was noted in Slovenia.