Can"t Stop Coughing? 8 Possible Reasons

What’s causing your chronic cough ?
You’ve been coughing for weeks. How do you know if it’s a hard-to-shake cold or something more serious?
Only a doctor can tell for sure what’s behind your hacking. A number of conditions could be to blame, such as asthma; postnasal drip; gastroesophageal reflux disease (GERD); and chronic obstructive pulmonary disease (COPD), a serious, progressive disease that includes both emphysema and bronchitis.
Read more about COPD and some common (and relatively uncommon) culprits in chronic cough. 
Asthma and Allergies
Asthma is a chronic lung disease in which the airways in the lungs are prone to inflammation and swelling. Along with chest tightness, shortness of breath, and wheezing, coughing is a characteristic symptom of asthma, one which tends to intensify at night or in the early morning. When the symptoms of asthma flare up suddenly, it’s known as an asthma attack.
Even in people without asthma, inhaling pollen, dust, pet dander, and other airborne irritants can trigger allergic rhinitis, an allergic reaction that can cause coughing, along with symptoms such as stuffy nose and sneezing.  
COPD
  COPD occurs when the airways and air sacs in the lungs become inflamed or damaged, most often due to smoking, and is more common after age 45. In COPD, the lungs produce excess mucus, which the body reflexively tries to clear by coughing. COPD-related tissue damage can also make you feel short of breath.
Your doctor may check you for COPD (particularly if you smoke), after ruling out other common causes of cough. To determine if you have COPD, your doctor is likely to conduct some tests, including spirometry, which involves inhaling as deeply as you can and then exhaling into a tube. 
GERD
GERD is an ailment of the stomach and esophagus that occurs when stomach acid backs up into the esophagus due to a weak valve.
The main symptoms? Killer heartburn. But coughing is another common symptom of GERD, along with chest pain and wheezing. In fact, GERD is a fairly common, and unrecognized, cause of a chronic cough. 
Respiratory Tract Infection
Coughing is one of the most common symptoms of colds and flu and other respiratory tract infections. A bad cough can outlast other symptoms (such as stuffy nose and a fever), perhaps because the air passages in your lungs remain sensitive and inflamed. When this occurs, it’s called chronic upper airway cough syndrome (or postnasal drip).
A more serious respiratory tract infection is pneumonia, which can be caused by bacteria or viruses. A cough, often producing a greenish or rust-colored mucus, is one of the characteristic symptoms of the illness, along with fever, chills, chest pain, weakness, fatigue, and nausea. 
Air Pollution
Various pollutants and irritants in the air can cause a persistent cough. Even short-term exposure to fumes

(such as diesel exhaust) can result in cough, phlegm, and lung irritation. Fumes can also exacerbate allergies or asthma.
Similarly, mold spores found in and around homes can cause wheezing and coughing when inhaled. In the wake of Hurricane Katrina, New Orleans reported a sudden spike in persistent cough complaints among returning residents. This so-called Katrina cough was believed to stem from the mold caused by the flooding, as well as by dry weather and the construction dust that was then ubiquitous in the city.  
Acute Bronchitis
If you’re recovering from a cold and suddenly develop a hacking, mucus-y cough, you may have acute bronchitis, a condition in which the passageways in your lungs become infected and inflamed. In addition to coughing and chest congestion, bronchitis can produce fever, chills, aches, sore throat, and other flu-like symptoms. These symptoms usually disappear within a few days, but the cough can persist for weeks.

If your cough doesn’t go away, or if you develop acute bronchitis frequently, it may be a sign of chronic bronchitis. Chronic bronchitis is a serious condition in which the lungs produce excess mucus due to ongoing irritation, and is considered a form of COPD. 
ACE Inhibitors
ACE inhibitors are a type of medication used to treat high blood pressure and heart failure. About 1 in 5 people who take the drugs develop a dry cough. In some people, the cough can persist for weeks after they stop taking the medication; women, African Americans, and Asians may be at greater risk of developing an ACE inhibitor cough than other people.
You should never stop taking a prescribed medication without consulting with your doctor, and ACE inhibitors are important medications for lowering blood pressure (a more serious condition than a cough.) Consult your doctor if you think your cough is related to a medication.  
Pertussis
Also known as whooping cough, pertussis is a bacterial disease with symptoms that include a slight fever, a runny nose, and, most notably, a violent cough that can make breathing difficult. Attempting to inhale air into the lungs between coughs can produce a distinctive, high-pitched whooping sound. After the initial stage, many people do not have a fever, but the chronic cough that accompanies pertussis can last for many weeks.
Although the number of pertussis cases in the United States has risen alarmingly in recent years, especially among adolescents and adults, pertussis is still a relatively uncommon cause of chronic cough.
References:
Health.com