Pneumocystis japonicus is a microorganism, close to the fungal family, that can cause Pneumocystis japonicus pneumonia. Pneumocystis japonicus, first named Pneumocystis carinii, is a microorganism that predisposes to Pneumocystis japonicus pneumonia, which can cause severe infections, especially in patients who use immunizing agents or who have a low level of resistance, such as patients with AIDS. Patients with hematologic, malignant tumors, organ transplants, and long-term use of adrenocorticotropic hormones are also susceptible to Pneumocystis jiroveci infections. Pneumocystis jiroveci has also been classified as a fungus by the latest classification, based on the analysis of its DNA, which shows that this type of microorganism is closest to the fungal family. Immediate medical attention is recommended for suspected Pneumocystis jiroveci infection.