Pneumocystis jiroveci, previously known under a different classification, is the organism responsible for causing Pneumocystis pneumonia (PCP). This organism is found globally, affecting both humans and animals. Serological studies suggest that most healthy children have encountered it by the ages of 3 or 4. Pneumocystis pneumonia tends to develop in individuals with weakened immune systems and in premature or malnourished infants. The symptoms of PCP include shortness of breath, a dry cough, and fever. Chest X-rays typically reveal infiltrates in both lungs. If left untreated, the progressive involvement of the lungs can lead to fatal outcomes.There is ongoing debate regarding how to classify Pneumocystis. Diagnosis is confirmed by identifying Pneumocystis jiroveci in samples obtained from induced sputum or bronchoalveolar lavage (BAL), which involves washing the respiratory tract. If these methods are not feasible, a transbronchial or open lung biopsy might be necessary. Additionally, the use of immunofluorescence with specific antibodies can detect the organism with greater sensitivity than standard microscopy. Treatment involves medication.