In considering the impact of epidemic disease on a society, historians usually conceive of their task in terms of problems of mortality and such associated discontinuities as the collapse of orderly government, the flight of threatened populations and the disruption of agriculture and trade. Important as these factors are, however, they tend to obscure the fact that, in terms of the perceptions of the peoples at risk, of no less significance is the fact that epidemic disease in the past has held, and today continues to hold, some of its worst terrors in the way it challenges the ideological structures that sustain all societies.

This is not simply a matter of explaining away incomprehensible horrors. The ideological underpinnings of a social system – whether in the form of political ideology, myth or religion – serve to rationalise the physical world in terms of the priorities, agenda and claims of the society generating these structures, and they comprise an ongoing discourse of self-definition that both responds to changes in social perceptions and historical circumstances and figures in the determination of how that society will react to any further changes or new developments. As these structures encompass the very essence and cohesive elements of a society – its sense of origins, identity, purpose and future – threats of the gravest and most disruptive kind are posed by challenges that falsify the assumptions and claims made in these structures.