The development and use of acyclovir accelerated in the area of the natural history, epidemiology, and biology of herpes simplex viruses. The development of serologic assays that differentiate HSV-1 from HSV-2 demonstrated the worldwide distribution of genital herpes, the role genital ulcers play in facilitating HIV transmission, and the high frequency in which pregnant women acquire HSV infection. The high reactivation rate of HSV-2 infections, the often silent nature of genital herpes, and its widespread anatomic distribution in the genitourinary tract lead to frequent subclinical transmission. Identification and counselling of the asymptomatic carrier is necessary if we are to decrease the transmission of genital herpes.