The inhabitants of Mexico City lived on a lake, often on floating or anchored rafts of vegetation and earth. It would be easy for people living like that to be contaminated by the water. The Mixtec, however, who were studied in this article lived in valleys near Oaxaca. Their neighbors, the Zapotecs lived on the hillsides so far as I could tell when I visited that area. It would be interesting to see if the Zapotecs also suffered major deaths for this disease. I imagine that Mixtec lands might be subject to flooding during raining seasons.
As we've spread out over the Earth, humans have remained fairly close to sealevel, that is, most people live within 1000 feet of it. Naturally, there are people who live much higher than that, and I don't blame them if it means more solitude. :^)