Before 1970, children in the United States were commonly vaccinated against smallpox. In the early 1970s, large-scale vaccinations of children against smallpox were discontinued because smallpox had virtually disappeared from the world. Recently, some scientists have cautioned that the Unite States could be exposed to smallpox again. Contrary to what many people believe, this exposure would probably find all age-groups within the United States population at comparable levels of risk of contracting smallpox, since ________.