Pneumonia Pathology

2021 Mar 8. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan–.


Pneumonia has been defined as an infection of the lung parenchyma. Rather than looking at it as a single disease, health care professionals must remember that pneumonia is an umbrella term for a group of syndromes caused by a variety of organisms resulting in varied manifestations and sequelae.

There have been many attempts to classify pneumonia based on the etiology, clinical setting in which the patent acquired the infection, and the pattern of involvement of lung parenchyma, among other classifications. This article reviews pneumonia based on the classification followed by the American Thoracic Society.

Community-Acquired Pneumonia (CAP)

Any pneumonia acquired outside of a hospital in a community setting.

Hospital-Acquired Pneumonia (HAP)

Any pneumonia acquired 48 hours after being admitted in an inpatient setting such as a hospital and not incubating at the time of admission is considered as HAP. This classification helps clear the confusion surrounding the terms healthcare-associated and hospital-acquired pneumonia. Now all pneumonia acquired in the setting of assisted-living facilities, rehabilitation facilities, and other healthcare facilities have been included under community-acquired pneumonia, and a hospital setting is necessary for classifying pneumonia as HAP.

Ventilator Associated Pneumonia (VAP)

Any pneumonia acquired 48 hours after endotracheal intubation is considered as VAP.

These categories have helped establish the common organisms responsible for each type of pneumonia and helped to formulate treatment guidelines for the efficient management in both in-patient and out-patient setting.

Depending on the pattern of involvement, pneumonia has historically also been studied as:

  1. Focal non-segmental or lobar pneumonia: involvement of a single lobe of the lung.

  2. Multifocal bronchopneumonia or lobular pneumonia

  3. Focal or diffuse interstitial pneumonia

PMID:30252372 | Bookshelf:NBK526116