Why we cough at all
The cough reflex is a protective explosion of air. Sensors in the airway lining detect irritation — dust, acid, mucus, infection — and trigger a deep breath, a closed throat, and a sudden burst that clears the airway at hurricane speeds. Most of the time we never notice it; it quietly partners with the slow, steady mucociliary clearance system that sweeps debris upward day and night.
Cough becomes a symptom when it is frequent, exhausting, or persistent. The single most useful question is how long it has lasted, because duration narrows the list of causes more than almost anything else.
Acute versus chronic
Doctors split cough by duration. Acute cough lasts under 3 weeks and is usually a viral cold or acute bronchitis. Subacute cough lasts 3 to 8 weeks, often the lingering tail of an infection. Chronic cough lasts more than 8 weeks and points to a small handful of common culprits: asthma, post-nasal drip, acid reflux, certain blood-pressure pills, and — in smokers — early COPD.
Sputum: dry vs productive, and the colors
A dry cough brings up nothing. A productive cough brings up sputum — the phlegm that is coughed up and spat out, an act called expectoration. Normal airways make a thin layer of mucus you swallow without noticing; sputum is what appears when that layer becomes thick, abundant, or infected. The amount matters: large daily volumes suggest bronchiectasis, where damaged airways pool secretions.
Reading sputum color (a rough guide, not a diagnosis) Clear / white : normal mucus, viral colds, ordinary asthma Yellow / green : many white cells -> infection OR brisk inflammation Rusty / brown : old blood, classically pneumococcal pneumonia Pink + frothy : fluid in alveoli -> think pulmonary edema Bright red : fresh blood = hemoptysis (see guide 3) Large volumes : daily cupfuls -> suspect bronchiectasis Note: green does NOT always mean bacteria. Color guides, the clinical picture and tests decide.