Syphilis is an infectious disease caused by the spirochete Treponema pallidum which penetrates abraded skin or intact mucous membranes easily and disseminates rapidly, although asymptomatically, via the blood vessels and lymphatics. It almost always is transmitted by sexual contact with infectious lesions. The chancre of primary syphilis develops at the site of transmission after an incubation period of 10-90 days, with a mean of about 21-28 days, and then heals spontaneously in 3-7 weeks. The syphilitic chancre begins as a single, painless papule that rapidly becomes eroded and indurated. The ulcer has a cartilaginous consistency at the edge and base. Chancres usually are located on the penis in heterosexual men, but in homosexual men may be found in the anal canal, mouth, or external genitalia. Common primary sites in women include the cervix and labia.