What causes a diaper rash in toddlers and infants?

The intestines have substances like digestive enzymes and bile acids that help break down food into small components that can be absorbed for nutrition by the body. These substances in the bowel movements can also break down the skin if a diaper is not changed soon after a bowel movement. During diarrhea these digestive forces are expelled from the body rapidly without being used up and can cause severe irritation to the skin.  Antibiotics can also lead to diaper rash because they often cause diarrhea and kill normal friendly bacteria that compete with yeast. That is why yeast diaper rashes are more common during antibiotic treatment.