Mycobacterium avium is everywhere - in both soil and water, and it can become airborne. I don't know for a fact, but I suspect that the reason it seems more prevalent in some places than others is either population demographics (age, susceptibility to infection due to underlying conditions, genetically predisposed people...) or climate (in warm climates, people are exposed to soil and water all year, versus it being covered by snow and/or people hibernating in cold places...) or both. Many water supplies contain the bacteria, below acceptable thresholds and not harmful to most people - except those of us with compromised lungs or immune systems. My pulmonologist will not even diagnose MAC unless two or 3 sputum cultures grow it because there is risk of environmental contamination with the bacteria through air or water (this happened when my husband & I were tested - only one of his 3 cultures showed MAC, all 3 of mine had it.