This problem is often worse in modern, tighter homes with little ventilation.
Molds are fungi, simple microscopic plants.
Mildew is a thin, black, or sometimes white, growth produced by mold. There are thousands species of molds. Outdoors, they live in the soil and help in the breakdown of organic matter. Molds grow indoors on virtually any substance when moisture is present. Molds can be usually detected by a musty odor.