I first began traveling to India in the 1980s, drawn by a fascination with this ancient country that cherishes its history and harbors great ambitions for the future. My interest was professional as well as personal. Microsoft was expanding, our need for talent was growing, and I was attracted to the vitality and ingenuity of the Indian people.

A few years later, several colleagues and I were flying into Bangalore. As we made our final approach, I looked out the window and saw an area of densely packed, tiny, dilapidated homes stretching out for miles. At that moment, one of my Indian companions declared proudly, 'We have no slums in Bangalore.' Whether out of denial or innocence, my colleague didn't see the 'other' India. I don't mean to single him out. It can be easy to turn our eyes away from the poor. But if we do, we miss seeing a society's full potential.

When Melinda and I started our foundation's work in India, we began to meet people from the areas we'd been flying over. They had little education and poor health, and lived in slums or poor rural areas -- the kind of people many experts had told us were holding India back. But our experience suggests the opposite: What some call a weakness can be a source of great strength.

In 1988, when there were approximately 350,000 new polio cases a year and the disease was crippling children in 125 countries, the World Health Assembly set the goal of eliminating polio world-wide. Progress came quickly. By 1994, the Americas were polio-free. Soon we saw the last case in China, the last case in the Pacific, the last case in Europe. By the year 2000, the number of polio cases had dropped by 99%. But the task of ending polio was not 99% done.

The remaining cases were concentrated in fewer countries, and India was one of the last nations left. This was no surprise. India's urban centers are among the world's most densely populated. Its rural communities are dispersed across a vast and often inaccessible terrain. The country suffers from poor sanitation. Its 1.2 billion citizens are highly mobile and give birth to 27 million new Indians every year. Experts predicted that polio would be eliminated in every other country before it was eliminated in India.

But India surprised them all: The country has now been polio-free for more than two years. India's success offers a script for winning some of the world's most difficult battles in every area of human welfare. The key has been the participation of the humblest, most vulnerable members of the Indian population.

To be successful, any campaign this big has to include a clear goal, a comprehensive plan and precise measurements of progress. But the antipolio campaign in India took a crucial extra step: It enlisted the support of the full sweep of Indian society, including health workers, ordinary citizens and some of the poorest people in the most impoverished regions of the country.

The heart of the plan was a simple and inspiring mission: to find the children. To defeat polio, it is essential to achieve up to 95% vaccination coverage in afflicted areas. There is no way to measure whether you're meeting that mark unless you know how many children there are, where they are and whether they've been vaccinated.

India responded to this challenge with an army of more than 2 million vaccinators, who canvassed every village, hamlet and slum. Vaccinators took the best maps they had and made them better. They walked miles every day and worked late into the night. They found children in the poorest areas of Uttar Pradesh and in the remote Kosi River area of Bihar -- an area with no electricity that is often flooded and unreachable by roads. They found the sons and daughters of migrant workers in bus stations and train stations, accompanying their families on their way to find work.

When Melinda and I visited India in March 2011, two months after the last case of polio was identified, we traveled to a brick kiln whose workers labored long hours at low wages and lived in mud huts. We met a young mother and asked if her children had been vaccinated. She ducked into her hut, retrieved a bag that held all her possessions, and rummaged around the bottom of it until she proudly produced an immunization card listing the names of all her children and showing that each had received the polio vaccine -- not just once, but several times. We were amazed.

Wherever India's vaccinators have gone, they've had help from local residents. In one Kolkata slum, a group of schoolchildren who call themselves the Daredevils have been relentless in this effort. Their community had never had house numbers, so the children assigned numbers. Using donated cellphones connected to global positioning satellites, they created a digital map, marking each house where children hadn't been vaccinated.

The fight to end polio is not over, not even in India, and new polio cases in the Horn of Africa and Syria underscore the importance of eradicating polio everywhere. Still, if the world maintains its funding and commitment, we can eradicate the disease globally within six years.

The accomplishments of India's vaccinators and children and politicians will not end when polio ends in their country. Now that they have found India's children, they can bring them and their families other vaccines, clean water, education, advice on maternal and child health, and support for agriculture -- all the things that people need to live healthy and productive lives.

Years ago, on that day we were landing in Bangalore, I didn't know nearly as much about India as I do now. I saw India's obvious talent and energy, but, like my colleague, I missed its hidden strength -- the rich, the powerful and the poor working together toward a common goal.