Mold spores are all around us including on our bodies, in the air, and in the soil. Mold spores are dormant and require adequate food, air, moisture and temperature to grow. Because mold spores can travel in the air, there is no practical way to prevent them from entering buildings.
Excessive levels of moisture will increase the likelihood of mold growth. The most common sources of excessive moisture are exterior leaks and high interior relative humidity from bathrooms, kitchens, humidifiers, etc.