Molds produce spores. These can be called tiny encased "seeds," that drift in the air. A contaminated house will have millions of these suspending in each cubic foot of air. The main source of molds in the house is the HVAC and its duct work. Ducts often turn leaky with time. Sometimes the joints are not tightened enough or they loosen up. Moisture trickles though these loopholes. The air that pass through the ducts bring with it host of dirt particle and debris. With these duct gets infested with matted layer of dust. Moisture also settles in. The result is a fertile ground for molds to breed and grow. Some molds are visible. You can identify them through it its discoloration and odor. You will see the discoloration at the vents that is a sign enough of active mold growth inside the ducts. A musty odor would also come out from the HCVAC.
Get a dehumidifier
If you live in humid climate, then you should get a dehumidifier fitted in the house. It will pull excess humidity from the air that will restrict mold growth.
Buy a powerful air filter
These days there are air purifiers that can combat the multiplying presence of mold. They are equipped with the HEPA air filter that can trap microscopic particles. These are very efficient and capture a minimum of 99.97% of pollutants at 0.3 microns. Mold spores range between one and five microns and will easily fall prey to the HEPA filters. You can fit these filters in spaces which are prone to molds such as basements, bathrooms, kitchens, and rooms with upholstered furniture. Replace the filters at regular interval.