Why are some molds dangerous?

By Susan Brewer, Ph.D., University of Illinois,
Department of Food Science and Human Nutrition
 

The spores can cause allergic, often respiratory, reactions in some people.  While molds grow on the surface of foods, heavy growth on the surface of food indicates that the roots have invaded the food extensively. They can produce compounds called mycotoxins.  Mycotoxins can be allergens, in which case, only people who are sensitive to them react to them (asthma, etc.).  Some are actually toxic in that they can damage the liver and other organs.  These toxins mold-produced compounds may spread throughout the food, so it is impossible to know where they are.
 
Source(s):
fsis.usda.gov.  “Safe Food Handling / Molds On Food: Are They Dangerous?”