Herpes simplex virus (HSV) is frequently shed after infection of the genital or perianal area. HSV shedding, as determined by culture, occurs on about 3% of days for immunocompetent women and men, and more for persons with HIV infection or if measured by polymerase chain reaction (PCR). Most horizontal and vertical transmission of HSV occurs during unrecognized or asymptomatic shedding, and the majority of HSV-2-infected persons are unaware of their infection. Many persons with 'asymptomatic' HSV-2 infection can learn to recognize genital signs and symptoms as recurrences of HSV-2 infection. However, some shedding episodes remain truly asymptomatic even after patient education. Antiviral therapy dramatically reduces asymptomatic shedding, and trials to evaluate its effect on HSV transmission are underway.