og verbal分册 遇到一段不会翻译，求大神指点

williamszu

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