In 1997, I came across a New York Times column by Nick Kristof that stopped me in my tracks. The headline was “For Third World, Water Is Still a Deadly Drink,” and it included a statistic I almost didn’t believe: Diarrhea was killing 3.1 million people every year—most of them kids under the age of five.

I didn’t know much about the problem back then, except that it seemed so solvable. After all, in rich countries it felt like it already had been. My oldest daughter was a toddler at the time, and we never worried that an upset stomach would kill her. None of the other parents I knew worried about that either.

But in much of the world, kids without clean drinking water or basic sanitation were constantly being exposed to rotavirus, cholera, shigella, typhoid, and more—dangerous pathogens that spread easily when toilets are scarce and water is contaminated.

Nick’s column ended up changing my life. I sent it to my dad with a note: “Maybe we can do something about this.” He agreed. And after he traveled to Bangladesh to see the problem firsthand, we made a $40 million investment in vaccine research for diarrheal diseases. That grant helped shape what would become the Gates Foundation—and kickstarted decades of progress that’s now saved millions of lives.

Within a few years, I stepped away from Microsoft to focus on this work full-time. Once you’ve seen what’s possible in global health, it’s hard to do anything else.

When we first got involved, diarrhea was one of the biggest killers of kids worldwide. But over the past two and a half decades, these deaths have dropped by more than 70 percent.

The biggest breakthrough came from making vaccines for rotavirus, the leading cause of severe diarrhea and death in kids, affordable and accessible. When the vaccines first debuted in the early 2000s, they were priced at around $200 per dose—which meant they were completely out of reach for most families in most of the world. So the foundation partnered with vaccine manufacturers in India like Bharat Biotech and Serum Institute to develop high-quality, low-cost alternatives. Today, rotavirus protection costs about a dollar.

But getting the vaccines developed was only half the challenge. The other half was getting them to the kids who needed them most.

That’s where Gavi came in. The organization was set up a few years earlier to help low-income countries pay for lifesaving vaccines that had existed for decades but weren’t reaching the world’s poorest. But they were well-positioned to do the same with a new vaccine, and they did—purchasing the rotavirus vaccine for millions of children and supporting countries as they added it to their routine immunization programs. USAID played a huge role in this work, too, by helping local governments train community health workers and strengthen their vaccine delivery systems. Meanwhile, public health campaigns promoted treatments like oral rehydration salts and zinc supplements that can save a sick child's life for pennies. (Think of it as the medical-grade equivalent of Pedialyte.)

As all this was happening, countries quietly made enormous progress on clean water and sanitation too. Since 1990, 2.6 billion people around the world have gained access to safe drinking water—and the number of people who now have basic sanitation similarly has skyrocketed. These improvements help break a cycle where kids get sick, recover, and then get reinfected a few weeks later.

Despite the incredible progress, around 340,000 kids under five are still dying from diarrhea each year.

Part of the problem is that many kids still don't get vaccinated. Some live in places where health systems are weak or vaccines are hard to transport and store. Others are caught in conflict zones that make it dangerous for health workers to reach them.

And new challenges make the fight against diarrheal diseases even harder than it was 25 years ago. Shigella—one of the nastiest bacterial causes of diarrhea—is becoming more and more resistant to antibiotics, and we still don't have a vaccine. Climate change is making cholera and typhoid outbreaks more frequent, as floods contaminate water supplies and droughts force people to drink from unclean, unsafe sources.

For malnourished kids, everything is harder: They're more vulnerable to diarrheal diseases in the first place, and their damaged digestive tracts don't respond as well to oral vaccines or treatments. For families barely scraping by, diarrhea is both a medical crisis and an economic disaster. Parents miss work to care for sick kids. Kids miss school. Expenses pile up. It's one of the ways that disease keeps families trapped in poverty—and one of the reasons that a country’s public health is key to its development.

The encouraging news is that there’s a promising pipeline of innovations that builds on what we already know and could save even more lives.

At the foundation, we’re supporting scientists who are working on a vaccine for Shigella, which has become the leading bacterial cause of childhood diarrhea. We’re also funding efforts to combine different vaccines into a single shot, which would lower costs and make things easier for health workers and kids alike.

New delivery methods could make a big difference too. One example: vaccine patches for measles that don’t require needles, refrigeration, or trained staff to administer them. Just peel, stick, and protect. 

We've already learned a lot about how chronic infections damage kids’ guts and make it harder for them to absorb nutrients or respond to vaccines. Now, scientists are researching how to repair that damage, which could help the sickest kids recover faster.

And outside the lab, environmental monitoring tools are being developed to detect early signs of outbreaks—by regularly testing sewage for typhoid, for instance. It’s like having an early warning system for epidemics.

We can’t afford to look away now

I’ve been talking about diarrhea for 25 years, even though it makes some people squeamish, because it’s a microcosm of global health. It’s proof that the world can come together to solve big problems. When we refuse to accept that some children won’t make it to their fifth birthday, we can save millions of lives.

But it’s also a warning of what can happen when we look away.

Right now, global health funding is being slashed around the world. According to one estimate, cuts to aid from the U.S. have already led to almost 60,000 additional childhood deaths from diarrhea. If nothing changes, by next January that number could rise to 126,000. These are projections, not final counts, but the reality is undeniable: When lifesaving programs are eliminated, kids pay the price.

Diarrhea is one of the most solvable problems in global health. We’ve come a long way, but we’re not done yet.