Our foundation began working in India a decade ago, at a time when many feared that the country would become a flashpoint for HIV/AIDS. Since then, we have expanded into other areas, including vaccines, family planning and agricultural development. In all of this work, Melinda and I have seen many examples of India's poor making dramatic contributions. But nowhere has this power been demonstrated more clearly than in the fight to end polio. Indeed, India's accomplishment in eradicating polio is the most impressive global health success I've ever seen.

I...