Far from the world’s fears about Ebola and MERS, a quiet revolution is taking place in the diagnosis of a disease much more prosaic but far more threatening: childhood diarrhea. After pneumonia, diarrhea is the deadliest threat to infants worldwide, killing about 700,000 every year.

More than 40 pathogens — viruses, bacteria and parasites — cause diarrhea in children in developing countries. According to decades-old guidelines from the World Health Organization, these children should receive oral rehydration; intravenous rehydration if they cannot keep fluids down; and a zinc supplement.

The guidelines also say that children should receive antibiotics only when there is blood in their stool. At the time the recommendations were written, the best science indicated that blood was a sign of bacterial infection, and bacterial diarrhea is usually the most dangerous.

But bacterial causes are more common than had been realized, recent studies have found, and blood is not always present. And because many children with diarrhea do not receive antibiotics under W.H.O. guidelines, some experts say, many die or end up stunted by persistent infections that could have been cured.