A few months back my wife told me something about polio that I never knew... Polio emerged as an epidemic after the introduction of sanitation. The reason is th

A few months back my wife told me something about polio that I never knew... Polio emerged as an epidemic after the introduction of sanitation. The reason is that polio doesn't have such a terrible effect on infants, so the entire population has been exposed to it as babies and had developed immunity. So while sanitation reduced a lot of other horrible diseases that come from contaminated water, it prevented the population from getting this natural immunity in childhood, leading to an epidemic. And the people who suffered the worst during that time were the more affluent people who would have had better sanitation available to them. Because even though your neighborhood might be clean, you simply couldn't avoid coming in contact with polio in the great mixing bowl of a town or city.