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.
どの国でもバラツキは大きく 居住地域の環境の何か ― 唯一の現実的な説明は恐らく 飲料水媒介による感染の度合いですが その違いが ある場所では有害な株を生み 他では毒性の弱い株を生んだのでしょう