MAC usually occurs hand-in-hand with bronchiectasis. We all know that the MAC bacteria exist everywhere. But only certain section of the population gets the infection while the rest are not affected. My understanding is that having bronchiectasis is one of the main causes of having MAC infection because our lungs are weekend by the disease and the damaged bronchial tube walls interferes with smooth travel of the phlegm up and down the tubes. When the phlegm gets trapped in the tiny pockets in the bronchial tube walls, the bacteria in the phlegm can fester. People with healthy lungs are unlikely to be infected for that reason. And for the same reason, we can be re-infected by the same bacteria even after being treated with drugs.