Pneumocystis jirovecii (formerly Pneumocystis carinii) is an ascomycetous fungus that causes opportunistic pulmonary infections in immunocompromised patients such as human immunodeficiency virus (HIV) infection, haematological/solid malignancies, transplant recipients and receiving chronic immunosuppressive medications. Although the reports of defects of cell-mediated immunity in patients with diabetes abound, diabetes mellitus is not considerable risk of developing Pneumocystis pneumonia. In this review, I discuss aspects of the biology of this pathogen and the damage it causes to the host lung, and then I summarize the clinical, laboratory, and radiological findings of Pneumocystis pneumonia in immunocompromised patients.