Chlamydia pneumonia infection is an infectious disease caused by Chlamydia pneumoniae. It mainly causes atypical pneumonia in adults and adolescents, and can also cause acute respiratory infections such as bronchitis, pharyngitis and tonsillitis. According to statistics, it is the third major pathogen causing community-acquired pneumonia after pneumococcus and Haemophilus influenzae. The United States reports about 300,000 cases of the disease each year, while there are only about 150 cases of pneumonia caused by Chlamydia psittaci. There are similar reports in Hong Kong. Among patients with respiratory infections, the positive rate of serum Chlamydia pneumoniae antibodies is 54.8%, and that of severe infections is 24.8%. The antibody titer is mostly ≥1:256, while the positive rate of Chlamydia psittaci antibodies is only 0.9%, and most of them are not recent infections. In addition, it has been found that Chlamydia pneumoniae infection is significantly associated with the occurrence of cardiovascular diseases such as coronary heart disease, myocardial infarction and dilated cardiomyopathy, as well as cerebrovascular diseases.