FOR MOST of this century, publichealth officials have relied on a few basic techniques to contain contagious diseases. They have assumed that serious infectious diseases can be controlled only by interrupting the chain of transmission. To do so they have attempted to locate carriers of the disease, to alert those who may have been exposed to it, to offer eounseling to prevent further transmission, and to ensure that infected individuals desist from all activities that might spread the disease. Those who persist in knowingly spreading the disease have been subject to incarceration.