One Kitten Of Litter Has Diarrhea.why?

Intestinal parasites, stress, food change, and bacterial overgrowth are the most common causes of diarrhea in kittens.  The diarrhea means that the kitten is not absorbing nutrients appropriately and is losing liquid through the intestinal tract.  Kittens are very sensitive to dehydration and hypoglycemia.  It is very important to resolve this so your kitten continues to develop normally.  Have the kitten examined by a veterinarian.