The aspergillus fungus is common. It can grow on dead leaves, stored grain, bird droppings, compost stacks and other decaying vegetation. Although most people are frequently exposed to aspergillus, infections caused by it such as a pneumonia or fungus ball (aspergilloma) are rare.

Some people, however, have an allergic reaction (hypersensitivity) to this fungus, which is called allergic bronchopulmonary aspergillosis (ABPA). It is characterized by inflammation of the airways (bronchi) or air sacs (alveoli). The disease may mimic asthma or pneumonia, and, in fact, most patients with ABPA have asthma as well.

Patients with asthma or cystic fibrosis are at highest risk for allergic aspergillosis.