The science nerd here again -
While I agree that it is prudent to avoid vectors of infection, I still find the research on the issue of NTM in showers to be inconclusive as to whether there is high risk of infection there.
While the study cited, https://www.pnas.org/content/106/38/16393.long, was somewhat alarming regarding the widespread incidence of NTM in shower heads, I found this paragraph especially interesting:
"...Showerhead biofilms and water are potential sources of aerosolized microorganisms. However, different microbes and biofilms have different qualities that can influence partitioning into aerosols. Indeed, we and others have shown that mycobacteria can be selectively aerosolized, possibly a consequence of their waxy, hydrophobic quality (3, 25). To determine the makeup of shower aerosol microbiology, we collected aerosols during 20-min unoccupied shower operations with three showerheads analyzed rRNA gene sequences and compared them with biofilm, water, and ambient bathroom air samples. Microbial constituents were reflective of feedwaters and not biofilm. It seems possible, however, that any initial pulse of biofilm components would have been extensively diluted by water delivered during the aerosol collection period, and so not detected..."
In other words, the very characteristic that makes NTM difficult to eradicate, the waxy biofilm, may also make it less likely to aerosolize in the shower.
While the study goes on to conclude that showers may be a vector of infection, the paragraph above doesn't support the specific conclusion, in my opinion. Instead, I think the conclusion "...We conclude that there is need for further epidemiological investigations of potential sources of NTM infections, including showerheads. The methods we use here provide an experimental approach for such investigations..." is much more important.
My takeaway from all of this is to exercise caution, minimize exposure to the initial blast of hot, steamy water from any shower head, and continue to my healthy life precautions. For those who are seriously concerned about water-borne infection, I would say shower head replacement or disinfection is one of those reasonable precautions.
Also, I find it very interesting that NTM are far less prevalent in well water - makes me happy that for half the year, that is my water source!