Idiopathic acute eosinophilic pneumonia (IAEP) is characterized by the rapid accumulation of eosinophils in the lungs. Eosinophils are a type of white blood cell and are part of the immune system. Signs and symptoms may include fever, cough, fatigue, difficulty breathing (dyspnea), muscle pain, chest pain, and acute respiratory failure. The term 'idiopathic' means the exact cause for the overproduction of eosinophils is not known. Possible triggers of acute eosinophilic pneumonia include cigarette smoking, occupational exposure to dust and smoke, and certain medications. Diagnosis of IAEP generally involves a bronchoscopy and bronchoalveolar lavage (BAL).