Cough is an involuntary reflex. It is not an illness, but rather a protective reflex. The reflex occurs when the membranous lining of the respiratory tract secrete excessive mucous which protects the airway from infections. This mucous collects and traps all the foreign particles which enter the airway. Coughing helps us get rid our throats of this mucous. This enables us to keep our airways open and also prevent further infections like pneumonia which can occur.