Four years ago, I visited India and saw again what polio does to children. I was in a slum in East Delhi, when I met a 9-month-old girl named Hashmin -- paralyzed by polio -- cradled in her mother's arms. She will never be able to do many of the normal things kids do because she has polio. Watching her was the strongest of reminders of the imperative of ending this terrible scourge once and for all.

The following year, in 2009, India had more polio cases than any other country in the world. But much has changed since then, and this Friday will mark a full year since the last case of wild poliovirus was detected in India. This is a huge milestone in the history of global health.

With a huge and growing population, hard-to-reach migrant communities, and sanitation and health conditions that limit the effectiveness of polio vaccines, this remarkable achievement in India marks clear progress in the fight against polio.

India really stepped up to the challenge on polio. The government of India funded its own eradication program. Twice a year, 2 million volunteers prepare 800,000 vaccination booths around the country -- at schools, hospitals, and community centers. They immunize more than 172 million children one by one. Working with partners like Rotary International, WHO, and UNICEF, they have built an impressive infrastructure for delivering health services to some of the most underprivileged children in the world.

India's story is proof that major health problems can be solved in the toughest places in the world. But the fight against polio is not over and we are at a critical moment in time.

In 2011, there were still a handful of countries with polio outbreaks. We must sustain funding to ensure a comprehensive immunization effort in India and other countries -- until there are no more cases. If we don't, the virus can spread back into countries where it has been eliminated, as it did last year in Russia and China.

Eradicating polio is a top personal priority and of the greatest importance at the foundation. Last year, I visited India again to see how the country was able to make such phenomenal progress. I also visited Nigeria and Chad to understand the challenges that lie ahead. I'm optimistic that we can vanquish polio forever if other countries choose to learn from India's success. Together, we can accomplish something amazing.

Bill Gates is the co-chair of the Bill & Melinda Gates Foundation. He regularly posts his thoughts about the Foundation's work and other projects on his personal website, The Gates Notes.

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