What is pneumonia primarily defined as?

Pneumonia is primarily defined as an inflammation of lung tissue where the alveoli, the tiny air sacs in the lungs, fill with fluid or pus. This process disrupts normal gas exchange and leads to symptoms such as cough, fever, and difficulty breathing. The accumulation of fluid in the alveoli is a key characteristic of pneumonia, which can be caused by various pathogens, including bacteria, viruses, and fungi.

While pneumonia can be caused by a viral infection, it is not exclusively defined that way, as it can also be triggered by bacterial or fungal infections. Chronic lung diseases like chronic obstructive pulmonary disease (COPD) involve persistent cough but do not encompass the acute inflammatory process seen in pneumonia. Similarly, an infection limited to the pleural lining is known as pleuritis or pleurisy and does not characterize pneumonia, which involves the lung tissue itself. The definition provided emphasizes the involvement of alveoli and fluid accumulation, which are central to understanding pneumonia's pathophysiology.