From the CDC: "How do you keep mold out of buildings and homes?
As part of routine building maintenance, buildings should be inspected for evidence of water damage and visible mold. The conditions causing mold (such as water leaks, condensation, infiltration, or flooding) should be corrected to prevent mold from growing.
Keep humidity level in house between 40% and 60%. Use air conditioner or a dehumidifier during humid months. Be sure the home has adequate ventilation, including exhaust fans in kitchen and bathrooms."