The most common cause of a lingering cough is post-viral airway hyperreactivity — the infection is gone but the airways stay twitchy for weeks. Other causes worth ruling out: walking pneumonia, undiagnosed asthma, acid reflux (cough is its only symptom in many people), post-nasal drip, and medication side effects (ACE inhibitors). We listen to your lungs, check oxygen, and work through the list.