Molds are fungi that are prevalent both indoors and outdoors where moisture is ever present. Although the exact number of species s not know, experts estimate anywhere from tens to hundreds of thousands are uniquely identifiable. Mold reproduce through airborne spores which make it easy to spread and are adaptable enough to lie dormant in cold, dry climates and can amazingly remain dormant for 130 years plus.
More information available at Center for Disease Control.