PNEUMONIA Definition: "Pneumonia is an infection of pulmonary parenchyma". (Harrison) OR "Pneumonia is defined as an acute respiratory illness associated with recently developed radiological pulmonary shadowing which may be segmental, lobar or multilobar". (Davidson) Etiology: Microorganisms gain excess to the lower respiratory tract in several ways: Aspiration from the oropharynx (most common). Inhalation of contaminated droplets. Hematogenous spread. Contiguous extension. Pathophysiology of Pneumonia: Pneumonia results from proliferation of microbial pathogens at the alveolar level and the hosts response to these microorganisms. Defensive Mechanisms against Pneumonia: The defensive mechanism against pneumonia involve: Branching architecture of tracheobronchial tree. Muco-ciliary clearance. Local antibacterial factors. Gag Reflex. Cough mechanism. Normal flora adhering to mucosal cells of oropharynx. Alveolar mac