Pneumonia can be classified in several ways, most commonly by where it was acquired (hospital versus community), but may also by the area of lung affected or by the causative organism.

The most common causes of CAP vary depending on a person's age, but they include Streptococcus pneumoniae, viruses, the atypical bacteria, and Haemophilus influenzae.

Hospitalized patients may have many risk factors for pneumonia, including mechanical ventilation, prolonged malnutrition, underlying heart and lung diseases, decreased amounts of stomach acid, and immune disturbances.

Pneumonia has historically been characterized as either typical or atypical depending on the presenting symptoms and thus the presumed underlying organism.

The resulting lung inflammation is not an infection but can contribute to one, since the material aspirated may contain anaerobic bacteria or other unusual causes of pneumonia.

Aspiration is a leading cause of death among hospital and nursing home patients, since they often cannot adequately protect their airways and may have otherwise impaired defenses.

[12] Most common pathogens are Pneumocystis jiroveci, Mycobacterium avium-intracellulare complex, Streptococcus pneumoniae, Haemophilus species.

Less frequent pathogens are Cryptococcus neoformans, Histoplasma capsulatum, Coccidioides immitis, cytomegalovirus (CMV), and Toxoplasma gondii.

Severe acute respiratory syndrome (SARS) is a highly contagious and deadly type of pneumonia which first occurred in November 2002 after initial outbreaks in China caused by SARS-CoV, which had almost disappeared by the month of May 2004 (it was later called SARS-CoV-1 to distinguish it from other similar viruses).

The discovery of x-rays made it possible to determine the anatomic type of pneumonia without direct examination of the lungs at autopsy and led to the development of a radiological classification.

This is useful because chronic pneumonias tend to be either non-infectious, or mycobacterial, fungal, or mixed bacterial infections caused by airway obstruction.

The advantage of this classification scheme over previous systems is that it can help guide the selection of appropriate initial treatments even before the microbiologic cause of the pneumonia is known.

introduced type of healthcare-associated pneumonia (in patients living outside the hospital who have recently been in close contact with the health care system) lies between these two categories.