The ‘precautionary principle’ is frequently invoked in arguments about both security and the environment, but remains enduringly controversial. No matter how it is formulated, it has to do with probabilities and thresholds for action. Sometimes, it is taken to mean that there need not be proof that something is harmful before it is restricted: for instance, in the case of genetically modified foods. Sometimes, it is taken to mean that there need not be proof that something be beneficiail before it is done: for example, with organic foods. Sometimes, it has to do with who gets the benefit of the doubt, in the face of inconclusive or inadequate scientific data.
This article from Orion Magazine provides some interesting discussion of how it pertains to health threats generally, with an anecdote about rabid bats as an illustrative example.
I am not sure if there is all that much of a take home message – other than that people behave inconsistently when presented with risks that might seem similar in simple cost-benefit terms – but the article is an interesting one.