Public health has always been a science in the service of politics—a discipline that translates evidence into collective action and knowledge into policy. But something qualitatively different is now under way. The next decade will not merely test the capacity of public health to respond to new threats, it will test the epistemic foundations on which the entire enterprise rests. The crisis we face is not primarily a crisis of evidence—there is more health research than ever—but of the authority of knowledge: who produces it, whose counts, and whether shared facts can still ground collective responses.