Gastroesophageal Reflux Disease (GERD): This is often considered the most likely cause of a chronic cough of undetermined cause, as stomach acid can reflux into the esophagus and irritate the throat, triggering a cough. Many patients with GERD do not experience typical symptoms like heartburn.

Postnasal Drip (Upper Airway Secretion Syndrome): This condition, often due to allergies or a cold, can cause a cough as the body tries to clear excess mucus from the nasal passages and throat.
Asthma: Cough can be a primary symptom of asthma, especially in cases where wheezing is not prominent. Asthma can be triggered by a variety of factors including allergens, exercise, and respiratory infections.
Chronic Bronchitis: Part of chronic obstructive pulmonary disease (COPD), chronic bronchitis involves inflammation of the bronchial tubes, leading to a persistent cough, often with mucus production.

Lung Cancer: Although less common, lung cancer can present with a persistent cough as an early symptom. It's crucial to investigate for this, especially in smokers or those with a significant smoking history.
Pulmonary Embolism: A sudden onset of cough, especially if accompanied by shortness of breath, chest pain, or other concerning symptoms, could indicate a pulmonary embolism, which is a medical emergency.
Tuberculosis (TB): A chronic cough, especially if accompanied by weight loss, fever, or night sweats, could suggest TB, a serious infectious disease that requires prompt treatment.
Heart Failure: In some cases, heart failure can lead to a cough, typically worse when lying down, due to pulmonary congestion.

Sarcoidosis: A condition characterized by the growth of granulomas (inflammatory cells) in various parts of the body, including the lungs, which can cause a cough among other symptoms.
Eosinophilic Pneumonia: A rare disease where eosinophils (a type of white blood cell) accumulate in the lungs, leading to inflammation and symptoms including cough.
Histiocytosis X (Langerhans Cell Histiocytosis): A rare disorder in which the body accumulates too many immature Langerhans cells, leading to a variety of symptoms including cough if the lungs are involved.
Cystic Fibrosis: Although typically diagnosed in childhood, some milder forms may not be diagnosed until adulthood, and can present with a chronic cough among other respiratory symptoms.