Chronic Obstructive Pulmonary Disease (COPD): Given the patient's long history of smoking (27 years, one pack a day) and chronic cough, COPD is the most likely diagnosis. The patient's symptoms of respiratory problems starting a month ago could be an exacerbation of COPD.

Bronchitis: The patient's chronic cough and recent worsening of respiratory symptoms could be due to bronchitis, either acute or chronic, especially given the smoking history.
Pneumonia: The patient's recent illness could be pneumonia, which would explain the worsening respiratory symptoms.
Lung Cancer: Although the patient quit smoking 20 years ago, his significant smoking history still puts him at risk for lung cancer, which could cause chronic cough and respiratory problems.

Pulmonary Embolism: Although less likely, pulmonary embolism is a potentially life-threatening condition that could cause sudden worsening of respiratory symptoms and must be considered, especially if the patient has risk factors such as immobility or recent surgery.
Tuberculosis (TB): Given the chronic cough, TB should be considered, especially if the patient has been exposed to someone with TB or has risk factors such as immunosuppression.
Heart Failure: Heart failure could cause respiratory symptoms, especially if the patient has a history of heart disease, and is a critical diagnosis not to miss due to its high morbidity and mortality.

Cystic Fibrosis: Although rare in adults, cystic fibrosis could cause chronic respiratory symptoms, including cough and recurrent infections.
Alpha-1 Antitrypsin Deficiency: This genetic disorder can cause COPD-like symptoms in non-smokers or those with a minimal smoking history but could also be considered in this patient given his symptoms and smoking history.
Sarcoidosis: This autoimmune disease can cause respiratory symptoms, including cough and shortness of breath, and could be considered if other diagnoses are ruled out.