Chlamydophila pneumoniae is a species of Chlamydophila, an obligate intracellular bacterium that infects humans and is a major cause of pneumonia. It was known as the Taiwan acute respiratory agent (TWAR) from the names of the two original isolates – Taiwan (TW-183) and an acute respiratory isolate designated AR-39. Until recently, it was known as Chlamydia pneumoniae, and that name is used as an alternate in some sources. In some cases, to avoid confusion, both names are given.