Thank you Seth for the info about medical grade shower heads. Did your health insurance not cover the costs? It's a hefty price tag.
And here's a question for all: I have a tankless on-demand water heater. I've turned it up to 135 degrees F as you and Sue suggest. When the water is in a tank that's always at 135 degrees, it kills the MAC. But when it comes on-demand, how does that work? After I've run the water to warm it up (and catch it in buckets to water the garden) and the water gets to 135 degrees, does the MAC die instantly upon contact with 135 degree water as its coming through my shower head?