Théo suffered from these problems particularly during the first few months after he was born; it still occurs now but the outbreaks are much less frequent than before and a lot less serious.
Between 0 and 3 months, Théo suffered from recurrent diarrhoea and colic. My pediatrician helped me to understand why: Theo is lactose-intolerant. During that period, due to all that diarrhoea, he suffered terribly from nappy rash. Now that I know that it is lactose that is making him ill, I know how to respond. I've found the right gestures to bring him relief.
My pediatrician advised me to exclude lactose from his diet but also any potential sources of irritation to the skin on his bottom: inappropriate skin cleansing products, excessively tight or insufficiently absorbent nappies... And most importantly, I systematically apply the right cream on that area as it soothes him and is also a good prevention gesture.