"It's a paradox of public health that being too clean can sometimes lead to disease. For centuries, infants were routinely exposed to the poliovirus in their unsanitary living conditions. Polio rarely causes paralysis in infants, partly because of the maternal antibodies still present in their systems. In the 1900s, many countries cleaned up their water and sanitation systems, and houses in more affluent communities were routinely cleaned, reducing exposure to microbes. By the 1930s, '40s, and '50s, new mothers, especially those in the middle class, had no antibodies in their immune systems to pass on to their children. When children were exposed to the poliovirus in later years, they were defenseless against the disease."
"It's a paradox of public health that being too clean can sometimes lead to disease. For centuries, infants were routinely exposed to the poliovirus in their unsanitary living conditions. Polio rarely causes paralysis in infants, partly because of the maternal antibodies still present in their systems. In the 1900s, many countries cleaned up their water and sanitation systems, and houses in more affluent communities were routinely cleaned, reducing exposure to microbes. By the 1930s, '40s, and '50s, new mothers, especially those in the middle class, had no antibodies in their immune systems to pass on to their children. When children were exposed to the poliovirus in later years, they were defenseless against the disease."
https://www.pbs.org/wgbh/rxforsurvival/series/diseases/polio...