vishalwin wrote:Poliomyelitis,for example, emerged as an epidemic in the United States in the twentieth century; by then, modern
sanitation was able to delay exposure to polio until adolescence or adulthood, at which time polio
infection produced paralysis. Previously, infection had occurred during infancy, when it typically provided lifelong immunity without paralysis. Thus, the hygiene that helped prevent typhoid epidemics indirectly fostered a paralytic polio epidemic



Above is an excerpt from an OG passage. I am unable to understand the bolded part. Can anyone please explain what does author want to say here.

I am unable to understand the wording.

The author is saying that by then, meaning in the twentieth century, "modern sanitation", meaning modern habits of keeping things clean, resulted in an ironic development. The cleanliness resulted in people not being infected with polio when they were infants, at which point infection does not cause paralysis. Instead people would eventually be infected when they reached older ages, at which point the infection would cause paralysis.