Over the last 23 years, $9 billion has been spent on the ambitious effort to try to rid the world of polioviruses. After years of setbacks, there is a real sense the scientific tools may finally be up to the task of getting the job done.

But as scientists plan for the intricate dance that will be the final steps of polio eradication, they worry about a small and puzzling group of people who in theory could undo the hard-fought gains of the polio program. These people excrete polioviruses for months, even years after being immunized with oral polio vaccine, making them a potential -- and invisible -- reservoir for the virus.

Some strong-stomached scientists around the world are looking for these people, sifting through samples of human waste collected from sewers in search of excreted polioviruses. Their goal is to both assess the scope of the threat and find these so-called long-term excreters. While doctors don’t currently know how to stop these people from shedding polioviruses, a search is on for drugs that would stop this process.

Helen Branswell is a Nieman Global Health Fellow. Her travel this summer was supported by the Pulitzer Center on Crisis Reporting as part of its partnership with Harvard's Nieman Foundation for Journalism designed to strengthen global health reporting.

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Helen Branswell is the medical reporter for The Canadian Press, Canada’s news agency. In a journalism career that has spanned three decades, Branswell has covered a range of subjects including...