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Health officials are working to make polio the second disease, after smallpox, ever eradicated from the globe. But while smallpox carriers were easy to find because everyone infected developed symptoms, only a tiny fraction of those infected with the polio virus ever contract the disease. So while no one in India is reported to have suffered from polio in a year, the virus, which travels through human waste, still could be lingering.

That’s why the country will not be certified as completely polio-free until it goes at least three full years without a case. And it is why public health advocates warn against complacency in the massive vaccination efforts.

“We are at a threshold. If we take a long step, we may be in trouble,” said Dr. Yash Paul, a pediatrician in the northern city of Jaipur who was a member of the Indian Academy of Pediatrics‘ polio eradication committee until it was dismantled last year because the academy felt it was no longer needed.

Dr. Paul also appealed to public health officials to begin switching from the oral vaccine, which is easy to administer but contains live virus that can cause the disease in rare cases, to an injectable vaccine that uses dead virus.

The last time a country came off the endemic list was Egypt in 2006. If India succeeds in getting removed from the list in the coming weeks, only Pakistan, Afghanistan and Nigeria will remain. All three saw a rise in cases last year over 2010, and Pakistan is suffering a particularly explosive outbreak, Mr. Aylward said.

In addition, 22 other countries that had eradicated the disease suffered new outbreaks. However, some of those outbreaks stemmed from polio imported from India, so getting rid of the virus here is expected to lessen such outbreaks in the future.

Mr. Aylward hopes India’s success will spur donors to dedicate more money to the polio fight, partly because full eradication could free up funds for other global health issues.

The WHO program needs another $500 million to fund operations for the rest of the year, and some programs could run out of funding by March, he said.

“If we fail at this point, it’s an issue of will,” he said.