Eosinophilic pneumonia also known as acute eosinophilic pneumonia (AEP). This is a rare condition wherein eosinophils, a type of white blood cell that is part of your immune system, increase rapidly and accumulate in your lungs. Eosinophils form in order to combat allergens, inflammation or infection. The cause of the disease is yet to be determined, although researchers are looking into the link between environmental factors and the development of the disorder.