Causes of Cough
The main cause is anything that irritates the respiratory passage. Maybe, a small particle of food to a micro–organism. Also irritant odors and smoke cause excess sputum production which then leads to cough.

A harsh or forceful cough can itself be an irritant. Coughing causes the airways to contract, repeated contractions lead to irritation of the upper airways, and this prolongs the cough.

A dry, non–productive cough can be a side–effect of drugs prescribed for other ailments.

A chronic, persistent cough should always be considered abnormal. The cough may be caused by smoking, allergies, asthma or chronic bronchitis, but it may also be an indication of emphysema, tuberculosis or lung cancer.

Diagnostic Procedures for Cough
Any persistent cough may be a symptom of an underlying illness. Your doctor will make the diagnosis based on the frequency, duration and severity of the cough, breathing difficulties, the type of material being coughed up (the color and consistency of the mucus or blood), and the presence and location of any accompanying pain, swelling or rash. The doctor may wish to obtain a chest X–ray and a sputum specimen to test for infection. Although some of the chronic, obstructive lung diseases in which coughing may be a symptom are incurable (emphysema, some types of lung cancer and occupation–related lung diseases for example), most are treatable.

Sinusitis
A productive cough in which the color of the sputum changes from white (which is normal) to yellow or green may signal infection.

Pneumonia
A cough that produces blood – or pink, yellow or rust–colored mucus – accompanied by chest pains, headache, fever and difficulty in breathing.

Measles or German measles
If you have a hacking cough along with a pink rash, muscle aches, fever and red eyes.