If there is moisture than there can also be mold. The coils of an air conditioner, a faulty humidifer of a central heating system, or just high humidity can give mold spores enough moisture to thrive.
Mold spores are naturally occurring and are an important part of the environment outdoors. Indoors these tiny particles can cause a great many problems. Spores can enter indoors on our clothes, on our pets, or come in through vents. They get blown around by heating or cooling systems until they find moisture, oxygen and a food source. In a small space of time the beginning of an infestation is underway.