Mold exists in all structures and can be considered Nature's 'Clean-up Crew' because it feeds on moisture and dead or decaying matter. Mold colonies produce milions of spores that float in the air and land on all surfaces and invade the HVAC system of the building if their is poor filtration, and can ultimately cause a multitude of health issues. Many building materials such as untreated wood, sheet-rock, MDF, etc. support mold growth. Even dust that has settled on these materials or furniture can be a food source for mold. Just like humans, mold needs food to survive, and it is pefectly happy eating your home, as well as you, if you allow it.
Health risks can arise when mold spore and mycotoxin counts reach higher levels in your indoor environment, and most people (including many doctors) do not realize the severity of long-term mold exposure. According to the EPA, all indoor mold growth should be removed promptly using the correct safety precautions and protocols, no matter what type(s) of mold are present in the home.
Not all molds are considered 'toxic' to the human body; however, about 25-30% of the population is highly allergic to most molds and will display different symptoms, depending on the person and severity of the allergy. For example, the Mayo Clinic put together a study several years ago that found that 93% of all chronic sinus infections were direclty caused by mold/fungal exposure. Unfortunately, most doctors in the US still think that sinus infections are caused by bacteria and treat them as such -- and in the worst cases, surgery is performed. However, usually the surgery is only a temporary fix, especially if the mold issues are not addressed at home or at work, and the patient often has to go back for more surgery 3-5 years later.