Here’s the link to the article I read about showerheads containing the Mycobacerium Avium bacteria=
My thoughts about this article is that the sample size (45 showers, 4 in rural areas) was too small and that the researchers confused correlation with causation. It seems to me that the lack of mycobacterium in rural areas seems to disprove that dirty showerheads on their own can be responsible for the bacterium. Instead it was most likely that the bacteria came from the water source in the urban areas. Most likely I think the bacteria came from bird feces, which can carry the bacteria, in the water source. I know that New York and Chicago, two of the cities in states where testing took place, have large populations of pigeons and it wouldn’t surprise me if somehow if their feces somehow ended up in the water source of these showers.