Light micrograph of the single-celled fungus Pneumocystis jirovecii (dark blue, ovals) in a lung tissue sample from a patient with pulmonary pneumocystosis. The fungus is present within the alveolar spaces (air sacs) of the lung. 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 pulmonary pneumocystosis, also known as pneumocystis pneumonia (PCP), include fever, cough 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 Grocott's methenamine silver (GMS) method.