A cough is a sudden, forceful expulsion of air from the lungs through the mouth. It is a reflex action that helps 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 mucus or phlegm. Coughing can be a symptom of conditions like the common cold, flu, or more serious illnesses such as pneumonia.