Pneumonia is an infectious disease of the lungs in general, which can be cured by antibiotics and other treatments. However, there are many times a part of pneumonia should cause us to be alert to lung cancer. Lung cancer, a malignant tumor of the lung, usually manifests as a lung mass. However, when the mass is very small, it may not be detected by imaging. Obstruction of any foreign body or mass in any bronchus, even if it is incomplete, can lead to inflammation of local lung tissue, i.e. pneumonia. If pneumonia occurs repeatedly in one area, one should be alerted to the possibility of foreign body or tumor obstruction in that bronchus. In a small bronchus, a tumor as small as 5 mm in size can irritate the patient with symptoms such as coughing, hemoptysis, and local inflammation. A lesion of this size cannot be detected by chest X-ray or chest CT. Fiberoptic bronchoscopy is highly recommended if there is cough and repeated pneumonia in the same area. Recognizing a certain relationship between pneumonia and lung cancer can help detect lung tumors early. Don’t treat multiple times of pneumonia in one site as a general disease. Malignant tumor is a common and multi-infection disease now, and the people should be alert enough.