[求助]请翻译一个句子

hsummer

Poliomyelitis, for example, emerged as an epidemic in the US 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.

尤其是划线部分，做何翻译，谢谢！