Aspergillosis is caused by a fungus (Aspergillus), which is commonly found growing on dead leaves, stored grain, compost piles, or in other decaying vegetation.

It causes illness in three ways: as an allergic reaction in people with asthma (Pulmonary aspergillosis - allergic bronchopulmonary type); as a colonization and growth in an old healed lung cavity from previous disease (such as tuberculosis or lung abscess) where it produces a fungus ball called aspergilloma; and as an invasive infection with pneumonia that is spread to other parts of the body by the bloodstream (Pulmonary aspergillosis - invasive type).

The invasive infection can affect the eye, causing blindness, and any other organ of the body, but especially the heart, lungs, brain, and kidneys. The third form occurs almost exclusively in people who are immunosuppressed because of cancer, AIDS, leukemia, organ transplants, high doses of corticosteroid drugs, chemotherapy, or other diseases that reduce the number of normal white blood cells.