So is Mold.
According to the Centers for Disease Control and Prevention, the most common indoor molds are cladosporium, penicillium, alternaria and aspergillus. Stachybotrys chartarum, often called toxic mold, is rare.
The moisture can lead to mold growth in 48 hours. The mold smell is from the gases that mold spores generate while digesting nutrients, like Yeast smells while fermenting.
Yeast is single cellular while mold is multi cellular.