In light of the apparent worldwide eradication of smallpox, one might reasonably ask whether the models just discussed have any real significance. The answer is emphatically yes. For further application, we note that there are other diseases for which there is a prevention but no cure. The basic ideas if not the precise details of the models might apply, for example, to poliomyelitis, influenza, or measles. History has shown that if the quantitative approach had been applied to smallpox vaccination policy in the United States or Western Europe twenty years ago, it might have led to a substantial decrease in the waste of human and monetary resources on unnecessary vaccinations. Obviously no one would seriously consider making a $150 million a year decision based on the results of models at the simplified level of our prototypes, even if there were no fatalities involved. Features that should be added to make the models viable as bases for changing public policy include the age structure of the population, the difference in mortality rates between the first and subsequent vaccinations, and the different exposure risks to various population subgroups.