 [ Symptoms ] when do i need to be concerned about diarrhea in an infant [ Disease ] there are a few more questions that i would need to ask how old is the infant and how long has the diarrhea been going on is the child running a fever diarrhea can be dangerous in an infant because it can cause dehydration rather quickly infants do not know they should be drinking water and eating to help replenish essential nutrients so if this has been going on for more than a day my suggestion is to call your doctor or health care provider immediately to let them know what is going on