You notice a musty smell in the basement. A dark, speckled pattern creeps along the bathroom ceiling. A patch of wall behind the sofa feels damp and looks stained. The word that comes to mind is unsettling: mold. Your instinct might be to grab a bottle of bleach and a scrub brush, but a deeper anxiety lingers. Is it safe? Will it come back? Is there more I can’t see? Mold is more than a...