Molds are multicellular fungi consisting of long filaments that combine to form visible colonies. They are ubiquitous in diverse environments, such as soil, rotting food, and damp indoor spaces, and they serve an essential ecological role in decomposing dead plants and animals. While some molds can cause allergies or produce disease-causing metabolites called mycotoxins, others have critical pharmaceutical uses, such as the production of the antibiotic penicillin and the immunosuppressant cyclosporine used to prevent organ rejection.