Pulmonary infections may be caused by bacteria, fungi, viruses, or mycoplasma. Bacterial infections generally result in a polymorphonuclear (neutrophil) response. Bacterial infection of the lung (pneumonia) results in consolidation of the lung, which may be patchy or diffuse. Patchy consolidation of the lung is seen in bronchopneumonia (lobular pneumonia), while diffuse involvement of an entire lobe is seen in lobar pneumonia. Histologically, bronchopneumonia is characterized by multiple, suppurative neutrophil-rich exudates that fill the bronchi and bronchioles and spill over into the adjacent alveolar spaces. In contrast, lobar pneumonia is characterized by four distinct stages: congestion, red hepatization, gray hepatization, and resolution Reference: Robbins & Cotran Pathologic Basis of Disease, 9edition.