Traditionally, CAP is thought to be caused by inhalation or aspiration of a respiratory pathogen into an otherwise sterile alveoli. The local inflammatory response to the pathogen results in pulmonary signs and symptoms such as cough, sputum production, dyspnea, crackles, and hypoxemia. Release of cytokines into the bloodstream leads to the systemic signs or symptoms of pneumonia, which often include fever, fatigue, tachycardia, and leukocytosis. With the discovery of the lung microbiome, the traditional model has evolved. When a respiratory pathogen arrives in the alveolar space, it likely has to compete with resident microbes to replicate. Additionally, resident microbes may also modulate the host immune response to the infecting pathogen. Hypothetically, CAP might also arise from uncontrolled replication of microbes that normally reside in the alveoli.
