Based on content from Johnson & Johnson's Baby Care Basics: The Essential Guide for New Parents.
Diaper rash begins when trapped moisture and friction begin to irritate the skin. The irritation continues over time, and the harsh urine and stools inside a soiled diaper can make the situation worse. Some babies can also develop a diaper rash when they are sick or teething. Diaper rash is not only uncomfortable for the baby, but if it is left untreated, or doesn't resolve itself, it can even lead to infection.