Special Pathology Pathological changes in disease conditions of selected systems: Respiratory system 🔬 Pathological Changes in Pneumonia Pneumonia is an inflammatory disease of the lung parenchyma, primarily affecting the alveoli and interstitial tissue. It is typically caused by infectious agents (bacteria, viruses, fungi) and leads to a series of progressive pathological changes in the lungs. These pathological changes can be best understood in four classic stages, especially in lobar pneumonia,