The underlying question is really why we all got bronchiectasis, as MAC and other pathogens (pseudomonas, fungi) are secondary to bronchiectasis. Environmental exposure to MAC is often the cause of MAC/NTM. Bronchiectasis settled in first. 50% of bronchiectasis cases are idiopathic. I've had an ENT surmise my bronchiectasis is some variant of CF, which has not been decoded yet. I have tested negatively for known CF variants but new ones are being identified. Who knows.