For my wife Melinda and me, the problem of global health inequity became visible 15 years ago, when we saw a simple pie chart in the newspaper breaking down the major causes of death among children. One of the bigger slices of the pie, representing 500,000 dead children annually, was labelled: rotavirus. I had never heard of rotavirus. Melinda had never heard of it. It turns out it is the leading cause of diarrhoea, preventable with a vaccine that only children in rich countries were getting. Our reaction was somewhere between disbelief and disgust. How could we not have seen even the barest outlines of this tragedy?

That rotavirus slice in the pie chart set us on fire. I was in my early-40s, Melinda was in her mid-30s, I was running Microsoft, and we were starting a family. We had plans to do philanthropy later, when there was more time. But all of a sudden it didn’t seem like there was any time to waste. We decided to do everything we could to get the vaccine out to every child who needed it. Now, twelve of the world’s poorest countries are already giving the rotavirus vaccine to children, and that number is scheduled to climb to 40 by 2015. The rotavirus mortality numbers are starting to come down. However, hundreds of thousands of children are still dying from a disease that is both preventable and treatable. The tension between how much the world has achieved and how much is left to achieve is the reason Melinda and I call ourselves “impatient optimists.”

Tonight, I will talk about the project I am spending most of my time on right now. It is the subject about which I am most impatient and most optimistic: the fight to eradicate polio. Most people in developed countries know of polio as a disease that used to paralyze lots of children. But it isn’t merely a historical curiosity—it still strikes children today.

We are working to wipe the virus off the face of the earth, and we have almost succeeded: There are only three countries in the world where the virus is still being transmitted. Fewer than 250 children were paralyzed last year. Stopping these last cases of polio in these last countries, however, is among the most difficult tasks the world has ever assigned itself. It is also among the most important.

The first conviction: When health improves, life improves, by every measure. Many skeptics will say, “What’s the point of saving a child from rotavirus if she can’t get enough food to eat, or read, or earn income?” It’s true that we need to invest in important areas like agriculture and education in addition to health, but it’s also naïve to act as if these issues are separate. Disease insinuates itself into every aspect of life. It can stunt brain development, inhibit the absorption of nutrition, and weaken the immune system forever. Our goal isn’t merely to stave off death. It is to help lift the heavy burden that sickness places on poor people’s existence, so they can seize opportunities in the worlds of school, work, and family. In fact, when parents are more confident their children will survive, they tend to decide to have fewer children, gradually bringing population growth down and leading to all sorts of beneficial effects.

The second conviction: Progress is already happening at an enormous scale. This is my all-time favourite chart. In the year I was born, more than 20 million children under the age of 5 died. Last year, that number was 6.9 million. Keep in mind that the world population keeps growing, so the improvement is even more impressive than it sounds. If the rate of death had remained constant since 1960, 31 million children would have died last year. The tens of millions of lives already saved inspire me to do more because they prove what’s possible.

 The third conviction: Vaccines work wonders. They prevent disease from striking, which is better than treating it after the fact. They are also relatively cheap and easy to deliver. Yet millions and millions of children don’t get them. This is still stunning to me. Before we started the foundation, we assumed all the obvious steps were already being taken, and we’d have to go after the difficult, expensive, or unproven solutions. In fact, our first big health initiative was devoted to delivering basic vaccines. I’m not saying it’s a simple matter to reach children with vaccines. It’s extraordinarily difficult. But universal coverage with today’s vaccines is achievable. It’s also possible to invent new vaccines in the future for diseases like malaria. Both these achievements would save millions of lives, and they are a major focus of our foundation.

Delivering one vaccine in particular, the polio vaccine is the top goal of our foundation. I’d like to turn now to that vaccine—and to the disease it prevents—and explain why they are my priority.

Polio has been with us for a long time. Archaeologists have found Ancient Egyptian carvings depicting people with withered limbs, walking with canes. Around the turn of the 20th century, increased urbanization and poor sanitation led to epidemics of polio that caused not just paralysis, but also extreme terror. When the virus appeared, typically in the summer months, it surged through cities and towns, filling up hospital wards with paralyzed children in a matter of days.

The Irish journalist Patrick Cockburn (Co-Burn), who was infected during a 1956 epidemic in Cork, has written about the panic that surrounded him that summer. Quote: “It was only on 13 June that the first case of polio was reported in Cork city and by early July the number had risen to six. For the first time reports of an epidemic began to appear in the local press. They were usually accompanied by subheadlines claiming that there was ‘No occasion for undue alarm’ and ‘Outbreak a mild one’. These repeated understatements somehow conveyed the very sense of fear which the newspapers were trying so hard to avoid. By the middle of July the number of children entering the fever hospital in Cork had risen to four a day…. People swimming in the river Lee, which flowed through Cork and received the full drainage of the city, were threatened with prosecution…. A month into the epidemic in Cork, many were convinced that not only were the dire facts of the epidemic being suppressed in the city, but that it had spread to Dublin where people were dying like flies in the fever hospitals.”

That 1956 epidemic was among the last in Western Europe. Jonas Salk’s polio vaccine had been approved the previous year. As soon as Albert Sabin’s oral vaccine became available in 1960, mass vaccination drove infections in developed countries down near zero. The fear, and eventually even the memory, of polio faded away.

Polio wasn’t gone; it just wasn’t so frighteningly visible in rich countries anymore. It became a disease of poverty. In 1988, the World Health Assembly passed a resolution supporting the global eradication of polio. In that year, the virus was circulating in 125 countries, and it paralyzed 350,000 people.

Within eight years of the resolution, most countries had built strong polio programs, and cases were down almost 90 percent globally. We made our first contribution in 1999, and based on the trendlines, we thought we were helping fund the final stages of eradication. In 2000, the virus was circulating in 20 countries. By 2003, that number was just six. In 2005, Egypt and Niger were declared polio-free, leaving only four endemic countries.

 But the virus is stubborn. In 2006, people in 13 countries that had already achieved polio-free status were infected by travellers from one of the four endemic countries. In several countries, as far-flung as Indonesia, Somalia, and Yemen, these importations led to large polio outbreaks. It would be five years before India celebrated its last case. And there are still three endemic countries left: Afghanistan, Pakistan, and Nigeria. I can say without reservation that the last mile is not only the hardest mile; it’s also much harder than I expected.

Let me try to express how hard it is to eradicate polio by comparing it to smallpox.

Every single person infected with the smallpox virus gets an unmistakable rash on the skin. The eradication strategy pioneered by Bill Foege was called ring fencing—as soon as you saw a case, you vaccinated aggressively in nearby towns to contain the virus. Polio, on the other hand, is transmitted silently. Only 1 percent of infected people show symptoms. The other 99 percent are contagious and don’t know it.

When symptoms do present, they are not tell-tale. They may start with a fever and headache. A few days later, ordinary muscle aches may get increasingly severe, and the patient’s reflexes can start to slow down. Only then does paralysis set in. The point at which a health worker sees a child with paralysis begins a two-week waiting period during which stool samples are collected, sent off to the lab, and tested. By the time the diagnosis is confirmed, the virus may have travelled hundreds of miles in any direction.

So ring fencing doesn’t work. Everybody, everywhere, is at risk at all times, unless they’re immune. Therefore, the only way to stop polio is to vaccinate a very high percentage of the population, leaving no reservoirs of susceptible people where the virus can survive. The threshold for what is called “herd immunity” varies by location, but it is never lower than 80 percent and can be as high as 95 percent. Achieving 95 percent coverage is very difficult. Even rich countries like the United Kingdom only vaccinate 95 percent of their people.  

How can much poorer countries possibly achieve a similar level of coverage? Take the example of India, the most recent country to eliminate polio. India started with the same approach as the United Kingdom—vaccinating children when they came into the clinic for routine visits. But too many Indian children never see the inside of a clinic. So the Indian government added a supply-side approach to the demand-side approach—that is, they started going out into communities, finding children, and vaccinating them house by house. Think about what this requires. India has more than a billion people. Geographically, it is 15 times larger than the United Kingdom. It also features some of the most severe terrain and weather in the world.

Behind me is a photograph taken in Bihar state during a flood in 2007. These health workers walked for miles in water up to their waist to vaccinate children living in a remote area along the Kosi River. Periodically, they’d reach a village like this one in the Madhubani District. Notice the vaccine box on his head. Not only do vaccinators have to track down every child, but the vaccines have to be kept cold the whole time. This process doesn’t happen just once. It happens constantly. Every child has to be vaccinated three or more times to ensure full immunity. And 75,000 children are born in India every single day. That is why India’s polio program employed 2 million people—almost entirely paid for by the Indian government.