[求助]OG RC41中一句没读懂的句子。

kuuiytt

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. 这句话前面和后面都看得懂，就是by then到paralysis之间，怎么看不懂到底说的是什么意思。请教NN给翻译一下。多谢！

kuuiytt

还是不明白。难道说现代的卫生设备条件使得只有青少年和成人才会感染并会导致瘫痪，而以前的时候婴儿期时候被感染但会产生终身免疫并不会造成瘫痪。这么说现代卫生条件岂不是起了反作用了？那不是很荒谬吗？