Light micrograph of the single-celled fungus Pneumocystis jirovecii (purple ovals) in a lung tissue sample from an AIDS (acquired immune deficiency syndrome) patient with 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 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.