Mold is a fungus from the same family that brings you yeast and mushrooms. It’s been around for ages. There are thousand types in the environment we live in, most go largely unnoticed, causing no problems. Some cure illnesses (penicillin), some ripen Brie and Camembert cheese, and still others develop the flavor of Sauvignon Blanc grapes. Others grow on our shower curtain, between bathroom tiles, underneath the sink, or lightly colonize on our cereals, breads, and fruit.
To grow, molds need a moist, relatively warm environment. They also need a food source, which is where your carpets, ceilings, lint, drywall, wallpaper, leather, clothing, insulation come in, serving as good nutrient source for growth. Though it is everywhere, not all types and levels are detrimental to property or individuals. The EPA puts out an excellent publication for homeowners and renters on how to clean up residential mold.