What is Mold? Molds are neither plants nor animals. They are microscopic fungi, related to mushrooms, yeast, and mildew—and they can be found everywhere. Fungi use enzymes to digest food and reproductive cells called spores to reproduce. Molds play an important role in the decomposition of leaves, wood, and other dead plant matter. Mold puts the "blue" in bleu cheese, and mold is the original source of penicillin, one of the earliest and most widely used naturally-occurring antibiotics. Unfortunately, mold is also one of the most common allergens on the planet.    Molds in the Environment Molds live in the soil, on plants, and on dead or decaying matter. Outdoors, molds play a key role in the breakdown of leaves, wood, and other plant debris. Molds belong to the kingdom Fungi, and unlike plants, they lack chlorophyll and must survive by digesting plant materials, using plant and other organic materials for food. Without molds, our environment would be overwhelmed with...