(Figs. A4-34, A4-35, A4-36, A4-37, A4-38, A4-39, A4-40, A4-41, A4-42, A4-43, A4-44, A4-45, A4-46, A4-47, A4-48, A4-49, A4-50, A4-51, A4-52, A4-53, A4-54, A4-55, A4-56, A4-57) One of the roles of the skin is to function as a barrier to the outside world. In this capacity, exposure to infectious agents occurs, and bacterial, viral, fungal, and parasitic infections may result. In addition, the skin may be secondarily involved by systemic infection and provides diagnostic clues to systemic infections such as meningococcemia, Rocky Mountain spotted fever, Lyme disease, and septic emboli. Most sexually transmitted bacterial and viral diseases exhibit cutaneous involvement; examples include primary and secondary syphilis, chancroid, genital herpes simplex, and condyloma acuminatum.