A cough is a sudden expulsion of air through the large breathing passages which can help clear them of fluids, irritants, foreign particles and microbes.

Irregular coughing is usually caused by a respiratory tract infection but can also be triggered by choking, smoking, air pollution,[1] asthma, gastroesophageal reflux disease, post-nasal drip, chronic bronchitis, lung tumors, heart failure and medications such as angiotensin-converting-enzyme inhibitors (ACE inhibitors) and beta blockers.

Chronic complications are common and include abdominal or pelvic hernias, fatigue fractures of lower ribs and costochondritis.

[5] A cough can be the result of a respiratory tract infection such as the common cold, COVID-19, acute bronchitis, pneumonia, pertussis, or tuberculosis.

The cause of the cough may be inflammation similar to that observed in repetitive stress disorders such as carpal tunnel syndrome.

Atopic cough occurs in individuals with a family history of atopy (an allergic condition), abundant eosinophils in the sputum, but with normal airway function and responsiveness.

Depending on how severe the asthma is, it can be treated with bronchodilators (medicine which causes the airways to open up) or inhaled steroids.

Chronic bronchitis is defined clinically as a persistent cough that produces sputum (phlegm) and mucus, for at least three months in two consecutive years.

The treatment involves anti-acid medications and lifestyle changes with surgery indicated in cases not manageable with conservative measures.

Coughing may be caused by air pollution including tobacco smoke, particulate matter, irritant gases, and dampness in a home.

People who exercise outdoors on hot, smoggy days, for example, increase their exposure to pollutants in the air.

A cough can be triggered by dryness from mouth breathing or recurrent aspiration of food into the windpipe in people with swallowing difficulties.

[17] Other causes of nocturnal cough include asthma, post-nasal drip and gastroesophageal reflux disease (GERD).

Capsaicin is what makes chili peppers spicy, and might explain why workers in factories with these fruits can develop a cough.

A voluntary cough, often written as "ahem", can be used to attract attention or express displeasure, as a form of nonverbal, paralingual metacommunication.

[19][20] Coughing, and huffing are important ways of removing mucus as sputum in many conditions such as cystic fibrosis, and chronic bronchitis.

[5] The cough reflex is initiated by stimulation of two different classes of afferent nerves, namely the myelinated rapidly adapting receptors, and nonmyelinated C-fibers with endings in the lung.

[26] A trial of antibiotics or inhaled corticosteroids may be tried in children with a chronic cough in an attempt to treat protracted bacterial bronchitis or asthma respectively.

[5] There is insufficient evidence to recommend treating children who have a cough that is not related to a specific condition with inhaled anti-cholinergics.

[citation needed] A cough is the most common reason for visiting a primary care physician in the United States.

[30] Domestic animals and vertebrates such as dogs and cats can cough, because of diseases, allergies, dust or choking.

[31] In other domestic animals, horses can cough because of infections, or due to poor ventilation and dust in enclosed spaces.

Deer can cough similarly to humans as a result of respiratory tract infections, such as parasitic bronchitis caused by a species of Dictyocaulus.