I won't bore you with the long scientific definition of mold all you need to know is it's a fungus. Mold spores are present inside and outside so you can not eliminate mold. Mold needs certain conditions for it to grow. There must be moisture present, oxygen and a food source. Our homes have two conditions present at all times for mold to grow. See our homes are built using wood, drywall,carpet est. so you always have a food source and of coarse you have oxygen and if you don't than you in for some real trouble. Add moisture from a plumbing leak, roof leak, a/c drain pan leaks, leaks around windows, at walls, or poor drainage around your home est. and you have the potential for mold  growth. Also very humid air can also provide enough moisture for mold to grow. So your a/c system provides more than just cool air for your home it also removes moisture from the air. If your buying a home and the a/c system has not been working or the home has been sitting vacant without the air running than you have conducive conditions and the potential for mold to grow. If the old owner has been using a humidifier this also can produce enough moisture in the air to allow for mold to grow. Mold likes to grow in dark damp areas and the temperature of in between 40 to 100 degrees F. Mold has to have water to grow. So if you can eliminate leaks and a high humidity level mold can not grow. Having a home inspection at your home or the home you are about to purchase can let you know if you have any conducive conditions present that can allow mold to grow such as moisture intrusion into the structure of your home or other leaks or a/c systems not functioning or fans venting into the attic space est. Our humid climate in Houston Texas can create some conducive and favorable conditions for mold to grow. I personally don't perform mold inspections, but during your Houston home inspection I will point out any conditions that are conducive for mold growth and note any mold like stains.