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Although genetic mutations in bacteria and viruses can lead to epidemics，some epidemics are caused by bacteria and viruses that have undergone no significant genetic change．In analyzing the latter, scientists have discovered the importance of social and ecological factors to epidemics．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. 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．Similarly, an outbreak of dengue hemorrhagic fever became an epidemic in Asia in the 1950's because of ecological changes that caused Aedes aegypti，the mosquito that transmits the dengue virus，to proliferate. The stage is now set in the United States for a dengue epidemic because of the inadvertent introduction and wide dissemination of another mosquito，Aedes albopictus．
6道题错了前两个。 文章的大体结构能看懂，但是三个例子是如何和第一句话关联的没看懂。
第一句理解为“虽然细菌和病毒的基因突变会导致传染病，但是一些传染病也由没有明显基因突变的细菌和病毒导致。
下面三个例子大体能看懂，只是不知道和第一句有什么关系啊。怎么表示出“一些传染病也由没有明显基因突变的细菌和病毒导致”？
另外，“Previously, infection had occurred during infancy, when it typically provided lifelong immunity without paralysis”这句如何理解啊？传染发生在婴儿时期，会提供终生免疫而不会瘫痪”？ | 
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发表于 2009-5-17 23:29:00
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