[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！

ppp1

是可能这么荒谬的. 不过据我所知,很多人得婴儿期时候被感染但会产生终身免疫,但是还是有很多人死掉啦.....这样就不用长大了再得病了.... 然而现代卫生条件可以让更多人终身不感染......这样就不是反作用了

himba

In analyzing the latter, scientists have discovered the importance of social and ecological factors to epidemics.。。。Another example is Lyme disease, which is caused by bacteria that are transmitted by deer ticks. It occurred only sporadically during the late nineteenth century but has recently become prevalent in parts of the United States, largely due to an increase in the deer population that occurred simultaneously with the growth of the suburbs and increased outdoor recreational activities in the deer’s habitat. 这是文中的另一个例子，说过去的程度没有现在的程度重，这与卫生条件是不相关的，看我贴的文章的第一句，说社会和生态是importance. 当然，delay exposure to polio 我也不是很理解，但是，按照作者的意思，肯定是现在比过去严重的。

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？

himba

A这是一个比较,in the twentieth century,直到成人才会被感染，而previously,在婴儿时期就被感染了