The precautionary principle and emerging biologic risks: lessons from human immunodeficiency virus in blood products.

In times of crisis, such as during the early 1980s when acquired immune deficiency syndrome (AIDS) was first recognized as a threat to the blood supply, it can be difficult to find reliable evidence upon which to base appropriate public health policies. Unreliable evidence produces substantial scientific uncertainty. Yet despite ambiguity and unanswered questions, decisions must be made and policy established to protect people's health. The precautionary principle provides important guidelines for public health policy decision making that are of particular value in times of crisis, such as the emergence of a new pathogen: be open and honest about scientific uncertainty; communicate with the public; and consider immediate, adaptable policy decisions. Ongoing research into the important uncertainties and review of policies in light of the data that emerge are crucial to the development of good public policy.