Why may diarrhea be a serious problems in infant?

yes it could be, diarrhea makes an infant dehydrated so make sure that the infant drinks plenty and also you should go and visit a doctor. I think the main issue is dehydration...diarrhea is diarrhea because it is loaded with water (i dont actually know why this happens) and all that water that is lost is obviously not used in the infants body. Another reason is that it hurts (as anyone who has ever had it can tell you) and contains stomach acid which can irritate or even cause damage to the skin.