As we all know from 9th grade Biology (you were paying attention, weren't you?), water is essential to all us flora and fauna, which includes the lower orders such as molds and fungi. Water moisture causes the mold and fungi to flourish. When mold and fungi want to reproduce, they throw off smelly spores (like pollen) into the air, that our noses recognize as musty. Besides smelling nasty, these funky spores can make allergy and asthma problems worse and can contribute to other health issues.