@pamelasc, an easy way to kill it in the showerhead is to fill a baggie with cleaning strength vinegar and twist tie it over the showerhead. Let sit for 3 hrs and remove. Gallon size baggie might be easier to use.

@pamelasc1 @windwalker Pam - Terri's vinegar solution is endorsed by the MAC specialists at National Jewish Health. At least in my home, it is certainly easier to do than removing the shower head and boiling (especially since I think repeated boiling would ruin the rubber washer/gasket thingies. It is important to use cleaning/industrial strength (7-10% acetic acid) as mixed results have been shown with household strength (3-5%.) I get mine from a friend who works in a lab, so don't know a retail source, sorry. I'm sure others here can tell you where they find it.
Terri - How often do you treat your shower heads? I admit my effort is hit-or-miss, kind of when I think of it, but possibly not often enough.
Sue