Most mold on bread is harmless. Some molds are poisonous though and can make you very ill.
This might help also :
Although mold can grow on many types of foods, mold on bread is common because bread provides a very desirable source of nutrients for mold. It’s often kept in a warm area of your home – a cupboard or breadbox rather than the refrigerator. It’s moist. And mold likes the ingredients in bread.