If bread is left without any cover in open air, no wonder the bread has to change its appearance while content will also change. Basically, when the bread remains unprotected in air, it gets bacteria and germs that infect it. The bacteria form brown or green mold which ruins the bread.
The molding of bread also occurs during manufacturing. Mold is a group of Fungi that occurs over the surface layer of the bread. Bread is a good source of food for several types of fungus due to which moldiness occurs soon over the bread. Mold always hunts for a suitable environment where it could use moisture and nutrients to continue reproducing. For reproduce, bread is the best place to survive and due to this fungus occurs on bread. Other than this, when fungi sit for long time on bread, a condensation occurs that make the bread moldy.
Unlike humans, mold has different process of metabolism and eating habit. Mold first digest and then eats. What is more interesting to know is that there are different forms of fungi that survive in the world and practices to eat everything excluding metal which they cannot eat and digest. Some of the selective breeding produces special fungi that are used as agents to cleanup specific compounds. Until all the nutrients available on the food are consumed, fungi keep reproducing. In fact, some forms of mold can even increase their weight as per hourly basis. The reproduction is done by means of spores. Spores are the reproduced tiny vectors. They are extremely small and are many. You can say there are around millions of fungal spores available in a room at one time.
However, there is nothing to get panicked as spores can easily be destroyed by cooking. This is why bread does not get infected immediately. It affects from airborne spores that flow onto the nutrition-rich surface and increase their numbers. They can even expand in cold conditions for example refrigerator. If the bread is at freezing point, fungi ultimately become dormant. Well, this does not mean that they are dead; they can be revitalized if exposed to heat and will continue to grow in numbers.