In every case there's a lot of variation, but something about the environment the people are living in, and I think the only realistic explanation is that it's the degree of waterborne transmission, favored the harmful strains in one place, and mild strains in another.
在每个病例中都有许多的变异， 但这与那儿的人们所居住的环境有关， 而且我认为水媒传播程度 是唯一正确的解释。 在一个地方对有害的菌株有利，而另一个地方则对温和的菌株有利。