A few days ago, I read some news that could help write a happy ending for a story that Melinda and I have been telling for years. The story is about a newspaper article that opened our eyes to diseases in poor countries. The article said 800,000 children died from diarrheal diseases every year. We were shocked. We thought, “That can’t be true. But if it is true, we should try to do something about it.”

Sadly, it was true. When we started our foundation in 2000, we made global health a priority.

As I learned more about the impact of diarrheal disease, I asked the foundation team that worked on them, “How many different pathogens cause diarrhea in kids? And which ones cause the most deaths?”

I was amazed by the answer. Diarrhea is caused by at least 40 different pathogens. Different ones have to be treated in different ways. But what really blew my mind was that it wasn’t at all clear which pathogens caused the most deaths.

The global-health community had never done a large-scale study on this question. The studies that had been done were limited, focusing on only very young children or looking at only a few causes of diarrhea. And that kept us from making as much progress as we wanted to. How could we know where to focus our efforts? Which pathogens should we go after first?

To answer that question, in 2007 the foundation supported a study that looked at 40 pathogens. It evaluated more than 20,000 children under the age of 5 in seven sites in sub-Saharan Africa and South Asia with high rates of deaths from diarrhea. The results just came out this month, and they’re very exciting.

The study, called GEMS (for Global Enterics Multi-center Study), found that most cases are caused by just four pathogens. Since we were dealing with 40 before, that means we’ve cut the number of potential targets by a factor of ten. And the global-health community already has, or is working on, vaccines for three of the four pathogens, including the most common one, rotavirus. That tells us we need to expand access to the current rotavirus vaccines and keep working on new ones that are effective in the developing world, which is a big focus for our foundation.

The study did contain one big surprise: At most of the sites, the second biggest cause was a pathogen called Cryptosporidium. Almost no one expected that. There’s very little research being done on treating or preventing it, so this is something the global-health community will need to explore.

There’s a lot of good news from this study. It confirms that we already have many of the tools we need to save a lot of lives, including vaccines and broader solutions like promoting access to clean water, exclusive breastfeeding, and other steps. And it points the way to new areas where we need to be working, like Cryptosporidium, to save even more children.

This study is another good example of why measurement is so important. (I’ve been writing about measurement a lot this year.) With this new data pointing the way, we can make sure every hour of effort and dollar of funding is targeted where it will do the most good.

The article about diarrheal diseases that Melinda and I read years ago inspired us to get involved in global health. It’s a sad story, but it doesn’t have to be. Progress like this makes me very optimistic that this story can have a happy ending. If you’re looking for reasons to be optimistic about saving lives, I can’t think of a better one than this.