Mold is a fungus that develops on humid or damp spots such as ventilation ducts, walls, shower cubicles, bathrooms, etc. You can identify mold growth either by smelling the musty, damp, or mildew odor or by observing signs such as water stains or discoloration on ceilings, floors, or walls in your house. Read more to learn how mold can make you sick and ways to prevent mold health risks.