Coughs are caused by flus and colds, which are triggered by viruses; allergies and asthma as the lungs react to allergens; irritants such as perfumes, cigar smoke and cold air; acid reflux when stomach acids irritate the trachea; sleep apnea; lung inflammation; and drug reactions, according to WebMD. A cough is a protective reaction, as it tries to remove particles that are foreign in the trachea or lungs.
WebMD explains that coughs are common in both healthy and unhealthy persons. A cough results from an internal or external trigger and functions to keep the windpipe and lungs clear. For example, during a common cold, coughing removes mucus from the nasal pathway, thereby reducing the viral load. Such a cough normally goes away after the cold, although sometimes it may persist due to irritation of the lungs.
A postnasal drip can trigger a cough, when mucus drips from congested nostrils into the throat. Allergens, stomach acids and other irritants are cleared from the trachea through coughing. If a cough persists for more than one week, it may be a sign of a serious medical condition, according to WebMD, Prompt medical attention should be sought if the cough interferes with normal breathing or sleeping or is accompanied by symptoms such as pain in the chest, fever and sweating at night, or expulsion of blood.