Pneumocystis jirovecii is a fungus that causes a serious lung infection known as Pneumocystis pneumonia (PCP). PCP primarily affects individuals with weakened immune systems, such as those with HIV/AIDS, transplant recipients, cancer patients undergoing chemotherapy, and people taking immunosuppressive medications.

PCP is treated with antimicrobial medications, most commonly trimethoprim-sulfamethoxazole (TMP-SMX), which is highly effective against Pneumocystis jirovecii. Other medications, such as pentamidine, atovaquone, or clindamycin-primaquine, may be used in cases of intolerance or allergy to TMP-SMX.

Polymerase Chain Reaction (PCR) tests can detect the genetic material of the fungus in respiratory samples, providing a highly sensitive method for diagnosis.