Reading Comprehension

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.