Molds are microscopic fungi that live on plant or animal matter.
No one knows how many species of fungi exist but estimates range from tens of thousands to perhaps three hundred thousand or more. Most are filamentous organisms and the production of spores is characteristic of fungi in general. These spores can be air, water, or insect-borne. Some people are sensitive to molds. For these people, exposure to molds can cause symptoms such as nasal stuffiness, eye irritation, or wheezing.