Fungi grow by releasing reproductive cells (spores) into the air, just as plants reproduce by spreading seeds. The airborne spores are invisible to the naked eye, which is a major reason mold is such a problem. It is not uncommon to find hundreds or even thousands of mold spores per cubic foot of indoor air. Spores are extremely small (1-100 microns)—20 million spores would fit on a postage stamp.
Spores can survive harsh environmental conditions, such as dryness, that do not support normal mold growth. In fact, many spores can lie dormant for decades until favorable conditions allow them to spring back to life.