Mold reproduces by forming tiny invisible spores that are completely invisible to the human eye. These spores can survive under almost any condition, and over a long period of time. When an airborne mold spore lands on a humid or wet surface, it can start growing or reproducing. It is the airborne mold spores that are causing illness to humans being exposed, as the spores are inhaled into the respiratory system. The spores are so small and light in size and can travel around the house contaminating areas where no actual growth is present. If you have severe mold growth in one area in your house, the chances are that other areas could be contaminated since spores can travel from heat rise, pressure differences, and through the HVAC system.