A 23-year-old man with HIV and a recent CD4 count of 512 cells/mm³ is seen for evaluation of fever and rash. On examination, he is noted to have a diffuse macular rash on his trunk, extremities, and palms (Figure 1). He has sex with other men and is taking suppressive antiretroviral therapy. He reports no neurologic or ocular symptoms, and the neurologic examination is normal. Syphilis serologic studies are ordered. Testing for syphilis was negative 4 months previously. A presumptive diagnosis of secondary syphilis is made.