Light micrograph of the single-celled fungus Pneumocystis jirovecii (purple ovals) in a lung tissue specimen from a patient with pneumocystis pneumonia (PCP). P. jirovecii spreads from person to person through the air. Patients on medications such as corticosteroids and immunocompromised patients are at the greatest risk of infection. Symptoms of PCP include fever, coughing and difficulty breathing. If left untreated PCP can lead to death. Treatment is most often with antibiotics such as trimethoprim or sulfamethoxazole. Sample stained by the Periodic acid?揝chiff (PAS) method.