Treponema pallidum, a spirochete, causes syphilis. It enters the body through mucous membranes and nonintact skin. Syphilis occurs in 3 stages. The primary stage is characterized by the chancre (see Fig. 87-1), a single painless ulcer with indurated borders that develops after an incubation period of 21 days on the penis, vulva, or other areas of sexual contact (including the vagina or cervix). The primary chancre heals and disappears after 3 to 6 weeks. The secondary stage occurs several weeks after the chancre disappears. Rash and lymphadenopathy are the most common symptoms. The rash starts on the trunk, spreads to the palms and soles, and is polymorphous, most often dull red and papular (similar to that of Pityriasis rosacea), but it may also take on other forms such as psoriatic or pustular lesions. The rash is not pruritic. Constitutional symptoms are common, including fever, malaise, headache, and sore throat. Mucous membrane involvement (“mucous patches”) includes oral or vaginal lesions, and condyloma lata, which are flat, moist, wartlike growths, may occur at the perineum, anogenital region, or adjacent areas (thighs). This stage also resolves spontaneously. Latency refers to the period between stages during which a patient is asymptomatic. Any patient with secondary or latent syphilis who presents with neurologic symptoms or findings should have a lumbar puncture and cerebrospinal fluid testing for neurosyphilis. Late stage or tertiary syphilis, which is less common (classically found in 33% of untreated patients), occurs years after the initial infection and affects primarily the cardiovascular and neurologic systems. Specific manifestations include neuropathy (tabes dorsalis), meningitis, dementia, and aortitis with aortic insufficiency and thoracic aneurysm formation.