Syphilis is infection caused by the spirochete Treponema pallidum. After decreasing dramatically since the widespread availability of penicillin in the 1940s, rates of infection have increased since the turn of the millennium in many countries, often in combination with HIV.  The organism is introduced into the anorectum during anoreceptive intercourse, and within 2-6 weeks causes a small papule (chancre) which eventually ulcerates. This is primary syphilis. In 10-20% of cases the primary chancre will be hidden within the anal canal, or it may be mistaken for a common fissure. Unlike chancres that appear on the external genitalia, anal chancres are quite painful. The chancres of primary syphilis regress spontaneously in 3 to 4 weeks, with the secondary stage appearing 2 to 10 weeks later.

In secondary syphilis, patients may present with nonspecific symptoms including fever, malaise, and aching joints.  A rash classically appears on the palms of the hand and soles of the feet. Alternatively, secondary syphilis can present as a pale brown or pink, flat verrucous lesion called condyloma latum. Symptoms of untreated secondary syphilis often resolve after 1 to 3 months; however up to 25% of patients will have a relapse in the first year. If left untreated, tertiary syphilis infection can result in spread to the central nervous system (neurosyphillis) or the heart. It can also be transmitted from mother to fetus resulting in congenital syphilis.

Diagnosis is by demonstration of the corkscrew-shaped yellow-green spirochetes on darkfield microscopic examination or biopsy of the rectal lesions. Duration of treatment depends on the stage at diagnosis and immune status of the individual, but often is given as one or multiple intramuscular injection of long-acting benzathine penicillin G.