Now efforts are focused on finding people known to have immunodeficiencies and asking them to submit stool samples for analysis. "But we have got only very few stool specimens till now and they all have been negative," Roivainen admits. "I don't know if we will ever be able to identify him or her."
Why bother, one might wonder. Even if Roivainen and her team find the long-term excreter, there is nothing medically that can be done to stop his or her virus-shedding problem. In some people shedding continues for decades. In others, it stops on its own. And in some cases, the viruses turn on the shedder, who develops polio.
Still, polio scientists are eager to get a better picture of who these long-term shedders are. That's because these people, some scientists fear, could potentially re-establish spread of polio if the global polio eradication campaign succeeds in stamping out wild polioviruses. Once wild polioviruses are gone, the World Health Organization will tell countries to stop using oral polio vaccine. Those that can't afford to replace it with the more expensive injectable vaccine will start to develop ever larger numbers of polio-susceptible children.
"I think they are a real risk," Tapani Hovi, a polio virologist who headed Finland's lab before Roivainen, says of vaccine-derived polioviruses. "Today still high vaccination coverage is preventing the problems which they potentially cause. But in future when the coverage will be decreasing, or even vaccination is stopping in some countries I'm afraid...the risks will be more realized."
While only about 40 long-term poliovirus excreters have been identified worldwide, no one knows how widespread the problem is. Hovi, for one, thinks there may be more of these people than scientists currently estimate.
He has known his share of long-term excreter frustration. During his time as lab chief, Hovi was involved in a search for a long-term excreter in Slovakia. (Finland's lab serves as the World Health Organization's polio reference laboratory for the five Nordic countries, the three Baltic states and Slovakia, Slovenia, Croatia and Austria.)
VDPVs had been found in sewage coming from Skalica, a town of 15,000 people located about 30 miles from Slovakia's capital, Bratislava. Working with sanitation engineers, researchers tracked the viruses to a branch of the sewage system that served five or six apartment blocks where about 500 people resided. Stool samples were collected from most of those residents. Analyzing that many stool samples is a lot of work, but the researchers thought they were on the verge of finding their man or woman. "And we couldn't find any polio virus in any of those (samples)," Hovi says.
It was later discovered two other buildings were illegally dumping into that sewage branch. But before further testing could be done, VDPVs stopped showing up in Skalica sewage samples. The trail went cold.
"It might have been possible to detect the person, but we didn't," Hovi says. "Perhaps he or she moved away. You never know. And also it is possible of course that he or she stopped shedding the virus."
This story was reported with a grant from the Pulitzer Center on Crisis Reporting. Follow @PulitzerCenter on Twitter