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.
In fiecare caz exista multa variatie, dar ceva din mediul in care traiesc oameni, si cred ca singura explicatie realista este aceea ca, gradul de trasnsmitere pe calea apei, a favorizat tulpinile periculoase intr-un loc si pe cele mai blande in alt loc.