Question

Moldy bread! Where did that come from? Moldy bread indicates that a fungus is growing in and on your bread. But how did it get there? The bread was in a plastic bag.

Answer 1

Mold spores float around in the air, so in many countries its basically impossible to keep them away from your bread. 

Answer 2

They were already there...

Step-by-step explanation:

The possible explanation is that the spores of fungus were already there. There is no doubt that plastic bag prevent even moisture but during packing of bread there is always some mishandling. The spores are always present in dormant state in air just waiting for a hostile environment to grow...

This is the reason why there is always a expiry date even on plastic packed bread...