Aspergillosis is caused by a fungus (aspergillus) that is commonly found in the environment. It grows on dead leaves, stored grain, bird droppings, compost piles, or other decaying vegetation.

It lung disease in three ways: as an allergic reaction in people with asthma; as a fungus ball (aspergilloma) in an old healed lung cavity from previous disease such as tuberculosis or lung abscess, and as an invasive infection with pneumonia that may spread to other parts of the body by the bloodstream (invasive aspergillosis). Only the invasive infection is called invasive pulmonary aspergillosis and is much more serious than the other types of aspergillosis. The invasive infection can affect any organ of the body, but especially the heart, lungs, brain, and kidneys. Late in the course of the disease, the nervous system, skin, and other organs may become affected.

Risk factors in addition to immunosuppression include a very low white blood cell count over a prolonged period. This is especially a problem in patients with prolonged neutropenia (low white blood cell counts) following chemotherapy or bone marrow transplantation.