First rule: Mold will not grow if moisture is not present. Airborn mold spores spread from the outdoors to the indoors landing on surface material. If any moisture or water vapors are present, mold will cultivate. If you see mold growing in your home, you already have a serious problem that must be addressed. Because the spores are microscopic, only large colonies are visible to the eye. The mold must be removed and the water problem fixed to prevent recurrence.
Information found at Environmental Protection Agency.