Pathogen typically enter the lungs through one of these routes:
- Microaspiration from the upper respiratory tract[2]
- Macro-aspiration of a large inoculum of bacteria[3]
- Inhalation of contaminated aerosols[2]
- Hematogenous spread (less common)[3]
- Contiguous spread from nearby infected areas[3]
## Host Defense Compromise
Pneumonia develops when the lung's defense mechanisms are overwhelmed or compromised. This can occur due to:
- Impaired immune response (e.g., HIV infection, chronic diseases, advanced age)[2]
- Dysfunction of defense mechanisms (e.g., smoking, COPD, tumors, inhaled toxins)[2]
- Altered mental status leading to aspiration[2]
- Disruption of normal flora by antibiotics or illness[3]