There are some widely used prognostic guidelines for CAP. These systems appear to be useful in assisting physicians to make more rational decisions regarding the need for admission [13] [14] [15] . Patient mortalities in the risk groups 3-5 on A-DROP and CURB-65, and IV-V on PSI have previously been reported as 11.5-23.3%, 11.6-21.0% and 12.5-29.2%, respectively [16] [17] [18] [19] . A striking fact is that the majority of the PCP patients were categorized as mild to moderate by these guidelines and resulted in respiratory failure, and poor outcomes. We emphasize that mortality prediction in PCP is not correct when these conventional guidelines for CAP are applied, even when PCP develops in the setting of CAP. Also, these guidelines definitely underestimate the severity of PCP as CAP. The important issue is why these guidelines cannot correctly estimate PCP severity. PCP without HIV infection shows quite different clinical pictures compared to PCP with HIV infection. PCP with HIV occurs slowly and gradually [20] . On the other hand, PCP without HIV is typically more acute and severe than when associated with AIDS [10] , often resulting in acute respiratory failure requiring a need for mechanical ventilation. We suppose that this results from the differences of pathologic mechanisms between PCP with and without HIV. It is evident that PCP without HIV is an allergic reaction originating from Pneumocystis jirovecii. Pneumocystis elicits many kinds of immune responses, including those by lymphocytes, macrophages, neutrophils, dendritic cells, and epithelial cells [1, 21] .