For polio, the hypothesis is that it was very common before 1900. I quote from "From Emergence to Eradication: The Epidemiology of Poliomyelitis Deconstructed"

For polio, the hypothesis is that it was very common before 1900. I quote from "From Emergence to Eradication: The Epidemiology of Poliomyelitis Deconstructed" http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2991634/ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2991634/ > In the pre-epidemic era, enteric infections were so ubiquitous that most infants were infected within 6–12 months, at a time when they had circulating antibodies passively derived from their nursing mothers. Although serum antibodies did not prevent enteric infection, they were sufficient to preclude viremia, thereby avoiding invasion of the central nervous system and paralysis. The result was the acquisition of active immunity under the cover of passive protection. So the hypothesis is that historically most people did have it and were immune, and only the improved hygiene past 1900 led to epidemics. We can indeed ask whether some individuals are worse off now because they didn't contract it in their infancy. But we're also doing pretty much the same thing with the vaccinations. Maybe we'll see a rise in allergic reactions when polio 2 vaccines are phased out. I don't think that anybody would advise infecting babies to get the benefits back though :-)