Mold is a type of fungus, and while some kinds are toxic, most are not. Mold is a normal part of the environment and causes little problem for those who are not allergic to it. Like all fungi, it reproduces by releasing spores. It's these spores that cause an allergic response to occur. In an allergic individual, the immune system over-reacts to the presence of what is really a harmless substance. Symptoms of a mold allergy include chest tightness, wheezing, and coughing.
This kind of fungi in the home is likely to be more problematic than that naturally present in the environment, because spores tend to be more concentrated when they are indoors. Also, certain home conditions favor the overgrowth of this organism. It likes moisture, enclosed locations, and high humidity. Many homes have some kind of water leak, which favors mold growth. Home inspections will reveal any water leakage and fungal overgrowth. If overgrowth is excessive, it can be remediated, or removed, from the premises.