Why do microbes not feed upon the living matters .Why only on dead and decaying

Microorganisms are known as decomposers which break down the organic material such as the dead animals and plants into simpler substances to be absorbed by them. They are not able to feed on living organisms because there is a defence mechanism present in our body known as the immune system. This protects us from the harmful attacks of microorganisms.When the living organisms die all the functions of the body stops working and no protective system works in the body. Therefore, microorganisms become active and decay the body.
Bacteria and fungi are the most common type of decomposers found in the environment.They live in the soil and feed on the dead and decaying matter.