Why are fungi and bacteria called destroyers?

Mushrooms are used in the food industry and pharmacology. Bacteria help us to digest food, to feast on some fermented milk products. In fact, bacteria are everywhere. But both fungi and bacteria bring not only undeniable benefits, but also harm. The fact is that mushrooms can destroy some materials. For example, mycelium crumbles a tree, making it fragile and brittle. That is why wooden houses need to be repaired frequently. The bacteria start the process of decay, which is also capable of destroying not only wood, but also more durable material.