The World Health Organization plans to announce on Monday that India has officially eliminated polio, one of the biggest public-health achievements of recent times, and one that could set the stage for stamping out the ancient scourge globally.

Public-health officials have been counting the days to the three-year anniversary of India's last recorded case, which allows the WHO to certify it as polio free.

Many long doubted that India could pull it off, given the country's size, poor sanitation and the enormous challenge of vaccinating millions of children, often in far-flung places and in the face of societal and religious resistance.

"India was by far the hardest place in the world to get rid of polio," said Bill Gates, chairman of Microsoft Corp. and co-chair of the Bill and Melinda Gates Foundation, a major financial donor in the global polio campaign. "It's quite phenomenal they did it."

Three more countries remain to be rid of polio; Pakistan, Afghanistan and Nigeria. Officials say it will be extremely difficult to achieve eridacation in these countries.