Factors governing speed of societal responses to threats: precision of definition of the threat may be more important than its anticipated likely severity.

Responses to two recent international public health threats, AIDS and BSE, both exemplify missed opportunities for early intervention. They are in contrast to the response to the Y2K computer issue, a threat of lesser-anticipated consequence but one that was more precisely defined. Lessons from these experiences should inform the public health response to new threats, such as the emergence of multiple drug resistance in human pathogens.