Whereas previous research on the disruptive effects of epidemic disease have focused on the ways in which epidemics affect social structure and function, this study focuses on the biological impact of social disruption. The hypothesis is that social disruption resulting from the occurrence of epidemic disease increases the biological impact of the epidemic, as assessed by disease incidence. This hypothesis is explored in terms of a "level of response" model, borrowed from Slobodkin and Rapoport (Q. Rev. Biol. 49:181-200, 1974). The Human Area Relations File (HRAF) provides ethnographic reports of social responses to epidemics. The most frequently reported response in the HRAF is flight or migration away from the locus of the epidemic, followed in frequency by extraordinary preventive and/or therapeutic measures and scapegoating. The model proposes a continuum of responses beginning with responses that are already part of the indigenous response to disease and proceeding through disruptive processes, including flight and rejection of authority systems. Social disruption increases the biological impact of epidemics by robbing the social group of important participants, dismantling public health programs, or producing general economic hardship. The model proposes a scheme for identification of situations under which particular social group responses are "biologically appropriate."