Diarrhea is not only a topic that makes people a bit squeamish, it turns out to be a difficult disease to put into numbers.

But two trends are clear, says an author of a new report: The number of deaths from diarrheal diseases is dropping dramatically in low-income countries – and ticking upward in wealthy nations.

An infection by E. coli, Cryptosporidium,Shigella or rotavirus, and the resulting diarrhea, is often a death sentence in much of the world. In 2005, about 1.6 million people died from diarrhea-related diseases, and roughly 770,000 of them were kids under 5. But that number has been steadily dropping, as a new study points out.

The study comes out of a research collaboration called the Global Burden of Disease group, a consortium of over 2,500 researchers working around the world to track major causes of illness and death, including diarrhea. Published this month in The Lancet, the study shows diarrhea-related deaths have declined about 20 percent from 2005 to 2015 for all ages to 1.3 million people, and 35 percent for children under 5 to about 500,000 children during the same time period.

"There are a lot of challenges on the horizon, but here's a case where there's been really quite steady progress," says Dr. Christopher Murray, lead author on the study and director of the Institute for Health Metrics and Evaluation.

But some experts caution against that the estimates may not be comprehensive. In a published response to the Global Burden Disease group's work, Boston University global health researchers commented good public health data are lacking in several countries included in the study. Places that have experienced recent conflict or upheaval like Liberia are less likely to have robust data collection compared to stable, high-income countries like Switzerland or Sweden, but all the countries' data are aggregated into global diarrhea estimate.

There are cases where "nobody really knows" what the diarrhea outcomes truly are, the Boston University researchers write. "Those comments are not unreasonable," says Dr. Richard Cash, a senior global health lecturer at Harvard University who was not involved with either article. "[The authors] are using some crappy data and good data, but they incorporate [all of it]."

That's a valid criticism, but ultimately unproductive, Murray says. "In some academic circles, but never in policy circles, people only want to pay attention to diarrhea in places where there's the most robust or highest quality data," he says. But if there's no data available, he says, policy people assume "that it's a zero problem" in that country. So Murray says the group did the best it could with the data that are available. Without even rough estimates built on shaky data, Murray says policymakers might be tempted to ignore the areas where diarrhea exacts the greatest toll.

And even if some of the estimates are built on shaky data, Cash says that the study illuminates an undeniable success in public health that began nearly half a century ago: "There were 4 million child deaths from diarrhea in the mid-'60s. The population of the world has just about tripled [since then.] If we still had those rates, we'd be seeing something like 12 million deaths, and now the number is roughly 1.3 million. It's remarkable."

Identifying what's responsible for these trends is difficult, says Manoj Mohanan, a public health researcher at Duke University. "We can conjecture that things like improvement in nutrition and economic statuses have led to better outcomes, but to pin down what's driving it causally is a lot harder."

Murray and his team give a few good guesses as to why diarrhea deaths have been on the decline, though. The rollout of the rotavirus vaccine in Latin America and Africa has likely contributed to fewer cases, Murray says. He also believes that "education of mothers has a huge effect on this." More mothers understand how to treat diarrhea cases, and therapies like oral rehydration packets are more widely available now, Murray says. "There are fewer cases coming into the hospital, and those that are coming in are less dehydrated than in the past," he says.

"There's also the ubiquitous availability of drugs and the contributing factors of [improved] water and sanitation — who can argue with that?" Cash adds.

There is one exception to the improving diarrhea situation, Murray points out. "There's a whole set of countries where diarrhea is getting worse [or more common]. It's the high-income countries, and it's because of the widespread use of antibiotics, perhaps inappropriately." The overuse of antibiotics, particularly in the elderly, has contributed to a rise in C. difficile infections, a deadly gut infection that can take root following aggressive antibiotic treatment.

But on the whole, Cash is optimistic. "These dramatic changes over a 50-year period are incredible and will continue," he says.

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