I think in my case it was a very logical assumption on the pulmonologist's part. I moved to a place at my winter home where there were feral chickens and many wild birds in my yard, gardened in the heavily contaminated and often dusty soil, and within 3 years had a raging case of MAC. And in addition I was an avid gardener up North, often working with peat potting soils, another big source.

With our always being outdoors and involved with the parts of the environment directly and indirectly with all the possibilities of what is in the air from the ground up and breathing it all in, especially on a windy days, we could be breathing in bacteria from the soil, etc. or breathing in other matter to cause MAI and not realize it.

For me....first I had two serious pneumonias at the same time in the 1980's, legionella (water based) and mycoplasma (air based). A good start for BE.

As I had mentioned in another thread I specifically have three incidents just prior to what I call "the straw that broke the camels back", for me.
First, I was cleaning out the backyard bird houses and specifically said to myself, "oh my gosh you just breathed in all that dust and debris.".
Second, I opened a bag of peat moss and particles raised up and I said to myself, "you just breathed all that in".
Third, I blew the winters dry leaves away from the foundation of the house and for sure there was dust, debris and mold that I did not realize how it could be affecting me as I blew it all up and away from the foundation of the house.
All this was just before I began to having to clear a substance from my throat all day long and carrying a spit cup to the doctors office and saying something is wrong.
True I believe I had had Bronchiectasis before the three things I did above in the yard. I also believe I had a MAC infection at least three years before I was diagnosed with BE/MAI because I was having to clear my throat in the mornings only and sometimes a colored piece of mucus would come up and out. I told the doctor and he said "not a problem."

It wasn't until after doing the three things I did above that I started having to clear my throat of a substance all day long. BE diagnosed by the same doctor only after I had lost 30 lbs. and he ordered the C Scan.
So for me I do believe it was all this that finally brought me to my having to deal with, and live with, BE and the MAI.

Just as the medical field is not sure of the why and how of many things but they have come to recognize the problem and all that has to be found out....we are not 100 per cent sure of how we brought the infection into our system and fortunately they have a way of knowing it is there.
Frustrating is the word for it all and learning to take further precautions and live with it, is the key. Hope so anyway.
Barbara