the scope of Federal intervention. However, in one circumstance, this same criterion would open the door wide to virtually unlimited immunization against influenza. That is the coming of another “killer” wave, another 1918. This is what was feared in 1976. But the threat was never established. We believe that in the absence of manifest danger, allout action was a mistake. One can, of course, start manufacturing more vaccine at the first hint of a killer. But one cannot reasonably stick it into people without more concrete evidence than anybody had at any time in 1976. To do so is to court medical dissent, to spread public confusion, and to provoke suspicion in the Washington community. Since research has not yet found a good predictor of virulence, one may have no means to establish in advance the severity of a presumed pandemic. Establishing that 1918 has come back again means waiting for manifestations somewhere in the world, maybe here. There is no way around it. Somewhere in the world, some people have to die. That is a challenge to medical research: how to predict virulence before the virus strikes.

For influenza, virulence and many other technical questions are important not only for future research, but also because current policy decisions turn on answers, or at least on expert guesses at the answers. Our next task, and the last in this study, is to sketch some of the technical dilemmas posed by flu; first, those related to the virus and disease, and second, those related to prevention and control. This we do in our Afterword. To the degree research unravels these dilemmas, influenza will become a far less slippery disease.