Pneumonia I: Introduction

Pneumonia is an infection of the lower respiratory tract that leads to inflammation of the lung parenchyma, often resulting in the accumulation of inflammatory exudate in the alveoli and airways. Unlike the watery, low-protein fluid exudate in pulmonary edema, the exudate in this case is a thick fluid rich in immune cells, proteins, and debris produced during infection and inflammation.This impairs gas exchange and can lead to consolidation of lung tissue. The infection may be caused by a variety of pathogens, including bacteria, viruses, fungi, protozoa, and parasites. In some cases, the infection can spread to the pleural space, causing pleuritis or parapneumonic effusion; if bacteria invade the pleura, it may progress to empyema.

Pneumonia is broadly classified based on the setting of acquisition: community-acquired pneumonia (CAP) and hospital-acquired pneumonia (HAP). CAP occurs outside healthcare settings and remains one of the leading causes of hospitalization, while HAP develops at least 48 hours after hospital admission. A subset of HAP, ventilator-associated pneumonia (VAP), arises in patients requiring mechanical ventilation and carries a high risk of mortality.

Risk factors for pneumonia include extremes of age (<5 years and >70 years), chronic lung or systemic disease, immunosuppression (e.g., HIV infection, chemotherapy), smoking, alcoholism, malnutrition, impaired swallowing, and residence in long-term care facilities. The use of medications such as benzodiazepines or gastric acid suppressants can also increase susceptibility.

The etiology varies with patient factors and clinical setting. Streptococcus pneumoniae remains the most common cause of bacterial CAP, while viral agents such as influenza and respiratory syncytial virus (RSV) are increasingly recognized, especially in immunocompromised or elderly populations. In HAP and VAP, multidrug-resistant organisms such as methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa are more prevalent.