Lesions on the back of a man with secondary syphilis. Syphilis is a sexually transmitted infection caused by the bacteria Treponema pallidum. The disease has three stages, first an ulcer at the site of infection followed several weeks later by a skin rash, headache and fever. The third stage, which follows a latency period that can last for years, involves spread to the organs and nervous system and can cause mental illness and death. Syphilis can be treated with penicillin if caught early.