Chlamydial pneumonia is a pneumonia cause by one of several forms of Chlamydial bacteria. The three major forms of Chlamydia responsible for pneumonia are Chlamydia pneumoniae, Chlamydia psittaci, and Chlamydia trachomatis.

In reaction to infection, infected lung tissue may become obstructed with secretions. As part of a generalized swelling or inflammation of the lungs, the fluid or pus secretions block the normal vascular exchanges that take place in the alveolar air sacs. Blockage of the alveoli results in a decreased oxygenation of the blood and deprivation of oxygen to tissues.

Chlamydia pneumoniae (in older literature known as "Taiwan acute respiratory agent") usually produces a condition known as "walking pneumonia," a milder form of...