Maybe allergies, maybe yeast infection, keep them dry, don't use wipes, let them go without a diaper, Butt paste, these are all great, but here is a strange one. My friend, as an adult, has psoriatic arthritis, a rare form of arthritis. She went to an arthritis meeting, and the first doctor that spoke said "severe diaper rash may be undiagnosed psoriasis". My friend had severe diaper rash as an infant. Does psoriasis run in the family? My friend's brother also has psoriatic arthritis.