There are estimated 400,000 species of mold species, or the Hyphomycetes. Hyphomycetes are defined to be filamentous and conidia (asexual spores) producing. However, a few hundreds common mold species frequently appear in our indoor environment and in the atmosphere.
During humid and warm summer, molds flourish everywhere, but for mold to grow, there must have 3 conditions met: humidity, moderate or warm temperature, and biodegradable organic materials to be present.
Besides molds are the producers of antibiotics, and emmitters of some microbial volatile organic chemicals and Nature's decomposer, almost all molds are allergens.
Some common outdoor and indoor molds are depicted in the following composite photo.