Persistent diarrhea for more than 24 hours in a child and three days in an adult is considered serious and requires medical attention, according to WebMD. Diarrhea combined with vomiting is more serious because dehydration can occur more quickly.
Uncomplicated diarrhea usually goes away within 48 hours without any medication, says WebMD. All that is needed is to stay hydrated. Vomiting can make the sufferer dehydrated in less time, particularly when attempts of oral rehydration is unsuccessful. Aside from dehydration, other signs of serious diarrhea that require immediate medical attention include blood in the stool, black stool, fever and excessive rectal or abdominal pain.