Northern Nigeria, October 2003. A polio outbreak was spreading through desiccated towns and remote border outposts, crippling dozens of children. As vaccinators went door to door in the blistering Ramadan heat, sermons rang out from local mosques. Imams railed against the misery of Baghdad, pulverized by American bombs; they condemned the pain of Gaza, under assault by Israeli helicopters. Pausing in front of the next compound, the vaccinators turned their heads. One man angrier than the rest was advancing towards them, his fist clenched. He pointed at the vaccine carrier, with its life-saving vials - shouting: "You are killing our brothers in Iraq and Palestine!" His finger rose, targeting the face of the vaccine team leader herself. "Do you now expect us to believe you come here just for our children's health?"

In those early days following September 11th 2001, we humanitarians were only beginning to realize how profoundly our world was to change. We stood on the brink of a new paradigm: where events on one side of the world would define possibilities on the other. Where our motives would be suspected, where our traditional shield - the mantra of impartiality, neutrality and independence - would shatter under repeated blows from anti-interventionist protests, aid worker kidnappings and summary executions.

It was the start of a global retreat for many humanitarian actors. Villages, towns - whole provinces - became no-go areas. Humanitarian compounds became like fortified bunkers. Some tasks became just too hard to manage, some risks too great to take. But polio eradication could not afford a retreat. Because that vaccinator team in northern Nigeria was standing in one of the very few places left where polio still hid, protected by a wall of implacable anti-interventionism. In fact, within a few years of that moment of challenge, polio and extreme anti-Western sentiment had converged on just a handful of locations - the poorest, the most excluded, and the most insecure.

It was the ultimate challenge for a programme that had deliberately styled itself in world-beating terms. The largest health effort on earth. The biggest development cooperation in history. A global public good. Polio eradication is indeed all those things.

And yet, for all its epic scale and historical ambitions, polio eradication is also - perhaps mostly - the story of the very small. Tiny pockets of resistance at the centres of great metropolises. Remote towns and villages where health workers don’t come. The handful of women and men who take a stand against the virus - even at grave personal risk. Press releases tout millions of children immunized and billions of doses of vaccine - but in the end, the only number that really matters to polio planners is the smallest: one. One suspected case. One infected child. One positive environmental test. One child left out of a campaign.

This obsession with smallness makes polio eradication invaluable - whether it succeeds or fails. Over-budget and over-time it may be - but we need polio eradication and its small victories, desperately fought and hard-won, if we are to achieve our other ambitions.

Why is polio so important? Because our global problems are not global at all; rather, they come down to very local issues.

So-called "Islamic terrorism" may cloak itself in a pan-national ideology, but it feeds on the rage of local exclusion: the shut-out rural poor of Deir Eizzor, disenfranchised Sunni communities across Anbar and Mosul, Karachi’s teeming urban slums. Likewise, pockets of poverty are far more pernicious and dangerous to countries than low national GDP. The gap that matters is not the one between the richest few and the rest of us, but between the rest of us and those who are so extremely poor that they hardly register in our lives - the ones who miss out on everything.

Science suggests that this equity gap is the most significant challenge to our hopes: for a fairer, more peaceful world, for national security and human security. It is also the question for which we seem to have fewest answers. Narrowing the gap, as development experts term it, is hard and expensive. It’s easy to avoid when there are lower-hanging development fruit within nearer and more comfortable reach.

Avoidance, deferring the hardest challenges, paralyses progress as surely as polio paralyses children. And in the long term, it’s a false economy. Studies have shown that investing in the poorest reaps extraordinary dividends, far outweighing any extra costs. One million dollars invested in the poorest few can reduce national mortality rates by 60 percent - according to UNICEF's 2010 study on inequity and development: Narrowing The Gap to Reach the Goals.

Polio had to learn this lesson early. After reaping the relatively easy victories of high-profile vaccination campaigns and celebrity ambassadors, it had to develop new strategies, specifically tailored around the last, hardest places. This was not a voluntary exercise. It was forced - by repeated failure to get vaccine drops into the same five percent, by bans on vaccination in outbreak zones, by becoming a target for national-local power struggles, by the retreat of almost every other aid effort as anti-Westernism surged, by the execution-style murders of almost 100 brave health workers.

But the rewards came, and not just in declining polio caseloads. Falling transmission rates are rightly celebrated, but polio knows - or should know by now - that they may just be respites and that reverses can easily happen. Polio's real success is reflected not in numbers alone, but in new, tentative intersections: between an internationalist human rights agenda and the world's most excluded, insecure communities.

And in some of these communities, polio has become the envoy for goals beyond itself. It has supported women’s rights to work and learn, provided access to basic hygiene, and held the door for emergency relief efforts. In very real ways, it continues to drag the rest of the world closer to cut-off areas - like the health workers in Bihar’s remote and often violent hinterland, dragging cold boxes miles over muddy floodplains to reach the Musa'aryi - a community so poor that mothers were forced to give birth on the riverbanks. Later, those cold boxes became makeshift waystations where vaccinators slept, then a permanent health centre, then a bridge across the river, where vaccinators now pass women in labour en route to clean, equipped delivery rooms.

How many development programmes have the tools, resources and pinpoint precision for such an effort in the poorest handful of places? Not nearly enough.

But polio eradication has created an approach outside the traditional development/emergency dichotomy. Unlike many development programmes, it seeks perfection: zero cases - and races against the clock to achieve it. It has no choice but to find the fastest, best, most efficient strategies - to hone in on the very communities, the precise local dilemmas representing the last milimetre to zero. And unlike so many emergency programmes that live and die based on attention-deficit news cycles and fleeting political will - eradiation has been able to play a long game, working over 30 years to refine and evolve.

Eradication may yet succeed - and soon enough. But that is not the real question. Instead we should be asking: how can this masterclass in reaching the unreached give us more than a polio-free world?

Even while polio still circulates, there's so much to learn from the eradication effort's small victories. Polio has shown us where we most need to concentrate - the communities most crucial to our common security and prosperity. It highlights the movement of people travelling in hope and fear, as the virus leaps from country to country. It has proven the importance of building trust before pushing agendas, and demonstrated how far we have to go in that regard. It has refined powerful analytic tools to make us evidence-based, efficient and responsive. And it has gifted us a legion of extraordinary women and men ready to deploy these tools in some of the world’s most desperate places.

If polio eradication succeeds - when it succeeds - the victory will belong to these frontline vaccinators, supervisors and communicators more than anyone. For the rest of us, a global win against polio should be a moment of deep humility. It took too long and cost too much. Not financially (the entire eradication budget is smaller than one year of US military spending in Iraq), but in lives lost and exclusion endured for too long. We should be ashamed that these communities were not reached sooner, and that it took our obsession with a virus to get us there. But if we can approach the Sustainable Development Goals with a fraction of polio’s precision and passion - then everyone’s biggest wins are still ahead.

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Claire Hajaj is a journalist specialising in conflict, policy, and advocacy whose experience spans strategic communication, international policy, national development, and humanitarian operations. She has written on the impact of conflict on polio eradication in countries experiencing internal struggles linked to Islamic extremist armed groups and other strains of fundamentalist Islam. She has worked on polio eradication at UNICEF and is the inaugural Fellow at the UN Centre for Policy Research.