When microorganisms move directly from an infected person to an uninfected person during sexual activity, bacterial disease can develop in the uninfected partner. Sexual activity between persons involves contact with the external genital and neighboring area, including the anus and surrounding perineal and perianal areas in both sexes.
Syphilis is caused by Treponema pallidum, a spiral bacterium, or spirochete, that has no Gram reaction. The bacterium functions in a low oxygen environment and is sensitive to dehydration. It invades the cellular lining of the urinary or reproductive passages, or the skin through small defects or breaks, and enters the circulation almost immediately. After about three weeks, the first cutaneous or mucosal lesion is a circular, surface wound called a chancre. It appears at the site of bacterial entry and progressively changes to form an ulcer. Extragenital chancres may appear around the mouth or anus. The lymph nodes draining the site of entry become swollen as well. These signs manifest the first stage of syphilis (primary syphilis). The chancre heals without treatment; and the lymph nodes return to normal size without treatment. Over this approximate nine week period, the spirochetes replicate dramatically without symptoms.
Untreated, syphilis enters the second stage (secondary syphilis) at about two weeks to six months, a condition characterized by appearance of new mucosal or skin lesions, signs and symptoms of a generalized illness (fever, sweats, headache, sore throat, malaise, weakness, and joint pain), and lymphadenopathy. Within two to six weeks, the signs and symptoms of secondary syphilis disappear. Yet infiltration of many organ systems by the spirochetes continue.
Tertiary (third stage) syphilis reflects a general systemic infection that develops over the next several years. It is characterized by various expressions of brain dysfunction, loss of movement-related sensation, generalized neurological deficits, skin and bone lesions, and cardiovascular disease.