"What we've seen in the past 10 years is, as soon as you relax your control measures, polio comes back in far greater numbers," Michael Toole, Deputy Director of the Burnet Institute in Melbourne and a member of the eradication initiative's independent monitoring board, told me. He points to outbreaks in China in 2011 as well as Tajikistan and the Democratic Republic of the Congo in 2010 -- all countries previously declared polio-free. In 2005, there was a polio outbreak in an Amish community in Minnesota, infecting five children but paralyzing none.

Like any virus, polio has to come from somewhere. The WHO uses genetic sequencing to trace the path of each case across communities, cities, and even national borders. But the fact that so few of the people who carry polio display symptoms can make it almost impossible to contain.

The resurgence of an old disease can be especially dangerous, as the world has learned before. In the 1950s and 1960s, the use of the insecticide DDT led to a reduction in the population of mosquitoes, which in turn decreased the number of deaths due to malaria. But the effects were temporary, and when the disease resurged, people had lost some of their natural immunity, and deaths spiked.

We've had similar warning signs with polio as well: the 2010 outbreak in the Congo, for example, had a 50 percent morbidity rate, WHO spokesperson Sona Bari told me, more than twice what is usually seen in unimmunized populations. "If we fail, we are not going to continue to have 50 kids paralysed each year, we're going to have hundreds of thousands," Aylward said.

But though polio is difficult to contain, it is looking increasingly possible to eradicate, largely due to the success in India last year. " I have a tremendous heart for India," says Sir Gustav Nossal, a renowned Australian immunologist who consults to the Gates Foundation. "If you go to Uttar Pradesh and Bihar, where the last pockets of the virus were in India, they are devastatingly poor. They have areas that are extremely inaccessible, that are flooded just about every single year and can't be reached for four months during the monsoon season. And yet, the Indians did it. They did it because of leadership and passionate commitment. That's what we now need from Nigeria, Pakistan, and Afghanistan."

Polio is sometimes framed as a moral issue: a question of whether all children have the right to safety from a deadly and debilitating disease, a safety that those of us in the wealthier parts of the world take for granted. And, to some degree, it is. As Rotary International's Carol Pandak puts it, "The specter of 200,000 children each year being paralysed by polio in the future seems unthinkable when you when there are resources available."

But the Western world has its own reasons to care, as well: a strong, proven, credible global health system, able to contain and eradicate diseases. Bill Gates, in his 2011 annual letter, called this "the rich world's enlightened self-interest."