Whatever may have caused the initial coughing, you can set up a chronic, self-induced bronchitis by not suppressing a cough to let the bronchial passages calm down. The mucus can be a result of irritation caused by.... the coughing.

But there's also a bug that lurks and lingers and is hard to find, that some folks I know have had, that needs long-term antibiotics to stamp out. I think that's condiered a form of walking pneumonia.

~Susan