[5] Chlamydia pneumoniae has a complex life cycle and must infect another cell to reproduce; thus, it is classified as an obligate intracellular pathogen.

[6] It also infects and causes disease in koalas, emerald tree boas (Corallus caninus), iguanas, chameleons, frogs, and turtles.

[7][8] Chlamydia pneumoniae is a small gram-negative bacterium (0.2 to 1 μm) that undergoes several transformations during its life cycle.

[12][13][14] In research into the association between C. pneumoniae infection and atherosclerosis and coronary artery disease, serological testing, direct pathologic analysis of plaques, and in vitro testing suggest infection with C. pneumoniae is a significant risk factor for development of atherosclerotic plaques and atherosclerosis.

[15] C. pneumoniae infection increases adherence of macrophages to endothelial cells in vitro and aortas ex vivo.

[16] However, most current research and data are insufficient and do not define how often C. pneumoniae is found in atherosclerotic or normal vascular tissue.

[19] Subsequent studies of bronchoalveolar lavage fluid from pediatric patients with asthma and also other severe chronic respiratory illnesses have demonstrated that over 50 percent had evidence of C. pneumoniae by direct organism identification.

[22] These observations suggest that acute C. pneumoniae infection is capable of causing protean manifestations of chronic respiratory illness which lead to asthma.

One of these RCTs[27] and another macrolide trial[28] suggest that the treatment effect may be greatest in patients with severe, refractory asthma.

Identification of immunogenic antigens is critical for the construction of an efficacious subunit vaccine against C. pneumoniae infections.