What causes Cough ? 

Coughing occurs when the nerve endings in your airways become irritated. Certain irritants and allergens, medical conditions, and medicines can irritate these nerve endings.
                
Irritants and Allergens
                
An irritant is something you're sensitive to. For example, smoking or inhaling secondhand smoke can irritate your lungs. Smoking also can lead to medical conditions that can cause a cough. Other irritants include air pollution, paint fumes, or scented products like perfumes or air fresheners.
                
An allergen is something you're allergic to, such as dust, animal dander, mold, or pollens from trees, grasses, and flowers.
                
Coughing helps clear your airways of irritants and allergens. This helps prevent infections.
                
Medical Conditions
                
Many medical conditions can cause acute, subacute, or chronic cough.
                
Common causes of an acute cough are a common cold or other upper respiratory infections. Examples of other upper respiratory infections include the flu, pneumonia, and whooping cough. An acute cough lasts less than 3 weeks.
                
A lingering cough that remains after a cold or other respiratory infection is gone often is called a subacute cough. A subacute cough lasts 3 to 8 weeks.
                
Common causes of a chronic cough are upper airway cough syndrome (UACS), asthma, and gastroesophageal reflux disease (GERD). A chronic cough lasts more than 8 weeks.
                
"UACS" is a term used to describe conditions that inflame the upper airways and cause a cough. Examples include sinus infections and allergies. These conditions can cause mucus (a slimy substance) to run down your throat from the back of your nose. This is called postnasal drip.
                
Asthma is a long-term lung disease that inflames and narrows the airways. GERD is a condition in which acid from your stomach backs up into your throat.
                
Other conditions that can cause a chronic cough include:
                
Respiratory infections. A cough from an upper respiratory infection can develop into a chronic cough.
                
Chronic bronchitis (bron-KI-tis). This condition occurs if the lining of the airways is constantly irritated and inflamed. Smoking is the main cause of chronic bronchitis.
                
Bronchiectasis (brong-ke-EK-tah-sis). This is a condition in which damage to the airways causes them to widen and become flabby and scarred. This prevents the airways from properly moving mucus out of your lungs. An infection or other condition that injures the walls of the airways usually causes bronchiectasis.
                
COPD (chronic obstructive pulmonary disease). COPD is a disease that prevents enough air from flowing in and out of the airways.
                
Lung cancer. In rare cases, a chronic cough is due to lung cancer. Most people who develop lung cancer smoke or used to smoke.
                
Heart failure. Heart failure is a condition in which the heart can't pump enough blood to meet the body's needs. Fluid can build up in the body and lead to many symptoms. If fluid builds up in the lungs, it can cause a chronic cough.
                
Medicines
                
Certain medicines can cause a chronic cough. Examples of these medicines are ACE inhibitors and beta blockers. ACE inhibitors are used to treat high blood pressure (HBP). Beta blockers are used to treat HBP, migraine headaches, and glaucoma.