“There’s something about the whole idea of eradicating polio that grabs the imagination,” says Aidan O’Leary. “Most people talk about making steps toward achievements, and it’s almost always into the never-never. Eradication is a zero-sum game; anything short of zero is failure. You keep getting closer and closer, but ultimately the only number that actually matters is zero.”

Although O’Leary, the polio eradication director for the World Health Organization, is speaking from his home in Galway, against the verdant backdrop of western Ireland, his focus is on war-torn Afghanistan and the parched and dusty plains of Pakistan — the last two places on the planet where wild polio still thrives.

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