Eczema is basically a type of allergic reaction either against an orally taken substance like dairy products, eggs etc. or a contact chemical like softener, soap etc. Whereas nappy rash is not allergic in origin, but is caused by moisture and the reaction against the chemicals in urine and stool. Nappy rash is also commonly caused by Candida infection as a result of prolonged exposure to the damp conditions in a nappy.