Four intestinal bugs are responsible for nearly half of all cases of childhood diarrhea, which kills about 800,000 children around the world each year.

That’s the main finding of a three-year project designed to give researchers and public health officials a clearer picture of a condition responsible for about 10 percent of deaths of children under age 5.

The four pathogens are a diverse group — one virus, one protozoan, and two species of bacteria. The leading cause — rotavirus, responsible for about 20 percent of cases — can be prevented by a vaccine that is only now starting to be used in developing countries.

In addition to mapping the microbial causes, the study looked at the consequences of diarrhea in young children. About 2 percent of children with “moderate-to-severe” cases died in the three months after they became ill.

“This had never been well quantified,” said Karen L. Kotloff, a pediatrician at the University of Maryland School of Medicine who helped lead the study. “It’s something that people had a sense was going on, but it had never really been measured systematically like this.”

The Global Enteric Multicenter Study, reported Monday in the Lancet, enrolled about 9,500 children under age 5 with diarrhea in seven countries. Four of the countries were in Africa (Kenya, Mali, Mozambique and Gambia) and three in Asia (Bangladesh, India and Pakistan). Most of the world’s childhood deaths from diarrhea occur in those two regions.

The children were all visibly dehydrated with sunken eyes and loose skin, or had been admitted to a hospital for treatment. They were compared to 13,000 healthy children from the same neighborhoods.

Rotavirus caused about 20 percent of cases overall and was the leading cause of diarrhea at most locations. Cryptosporidium, a protozoan, was the second most common cause. Escherichia coli and Shigella bacteria were the third and fourth. Together, they caused about 40 percent of cases of illness in the study.

Cryptosporidium is most commonly associated with illness in people with suppressed immunity, especially AIDS patients. But it caused illness widely, in places with both high and low rates of HIV infection.

“It surprised us how prevalent and important it was,” said Kotloff, who is associated with the Center for Vaccine Development at the University of Maryland.

Mortality in the children with diarrhea in the three months after their illness was nine times higher than in the healthy children. They also fell further down the growth curve, at least for a few months.

“Kids living in these poor environments are already fragile, and when they experience an episode of diarrhea, it really appears to have a major impact on their health and well-being,” Kotloff said.

Surprisingly, access to clean water did not differ significantly between the ill children and the healthy ones.

American children have received an oral rotavirus vaccine since 2006. GAVI Alliance, a public-private partnership formerly known as the Global Alliance for Vaccines and Immunization, is helping low-income countries get access to it. The organization hopes 40 countries in the developing world will have it by 2015.