The Aspergillus fungus is common in the environment, where it grows on dead leaves, stored grain, compost piles, or other decaying plant matter. There are three ways Aspergillus causes disease in humans.

Causes of aspergillosis include:

• Allergic reaction to Aspergillus: common in people with asthma
• A prior lung infection, such as an abscess, becomes colonized by Aspergillus and produces a fungus ball, called an aspergilloma.
• Invasive infection: infection starts as pneumonia and then spreads to other parts of the body. Infection can spread to the eyes, heart, brain, and kidneys. This occurs in those with weak immune systems.