Pneumocystis jirovecii is a fungus that primarily affects the lungs, causing a type of pneumonia known as Pneumocystis pneumonia (PCP). It is commonly found in the environment and can be inhaled, but it usually does not cause illness in healthy individuals. However, it can be dangerous for people with weakened immune systems, such as those with HIV/AIDS or undergoing chemotherapy. Diagnosis of Pneumocystis jirovecii infection typically involves imaging tests like chest X-rays and laboratory tests on lung fluid. Treatment often includes antibiotics, such as trimethoprim-sulfamethoxazole, which can effectively combat the infection. Preventive measures are also available for high-risk individuals to reduce the likelihood of developing PCP.