A cough is a sudden expulsion of air from the lungs that clears the air passages. It may indicate a respiratory infection or bronchitis or pneumonia or tuberculosis. Smokers may cough very often. Coughs in children are common, and usually accompany many of the normal childhood illnesses. A cough that continues for several days, or increases in severity, should be immediately checked by a physician to rule out other diseases. It is recommended that children that have a lot of mucus with their coughs cut back or eliminate dairy products from their diet, as dairy often adds to the production of mucus.