Molds are simple microscopic organisms. Almost any natural material will support a population of molds. They are found on plants, foods, dry leaves, and other organic matter.
Molds have an important role in breaking down organic material.
Molds are essential to what biologists call nutrient cycling. Nutrient cycling is the process where nutrients get used again and again by living organisms. Molds commonly release spores in the air that can cause allergic reactions in many people. For some people, a relatively small number of spores can cause illness. However, for other people a larger number of spores must be present to cause illness.