If you have MAC in your lungs, something is not normal in your airways. 90% of the time I breathe fine too, but...MAC only can take hold and grow if there are pockets of mucus for it to hide in - if your airways expel all the mucus in the normal course of the day, there is no home for it. If the CT shows MAC - either nodules or cavities, you need help to expel it. And it takes many months, if not years, to grow enough so you test positive for it in a culture.

Here is what Dr McShane, one of the foremost experts in treating NTM (non-tubercular mycobacteria) of which MAC is one type, has to say on the topic:

You might want to discuss this, sooner rather than later, with your doc.
Sue