A cough is a sudden, forceful expulsion of air from the lungs, often accompanied by a sound. It serves as a reflex to clear the airways of irritants, mucus, or foreign particles. Coughing can be caused by various factors, including infections, allergies, or environmental irritants. There are two main types of coughs: dry and productive. A dry cough does not produce mucus, while a productive cough brings up phlegm or mucus. Coughing can be a symptom of conditions like the common cold, flu, or more serious illnesses such as pneumonia.