"Once you have beaten back a disease to just a few hundred cases, they will almost by definition be concentrated in places where there's some barrier—geographical, cultural, political—to easy vaccination. In general, each marginal case will cost more, and will consume more time and effort and labor, than the one before it...[but] the math of cost-benefit analyses runs aground when it comes to eradication campaigns, because the benefits, in theory, are infinite."
In 1988 there were 350,000 cases of polio worldwide. Last year there were 223. But getting all the way to zero will mean spending billions of dollars, penetrating the most remote regions of the globe, and facing down Taliban militants to get to the last unprotected children on earth.
The border regions between Pakistan and Afghanistan are wracked by violence, and their rural hinterlands are largely under the control of a diverse array of militant groups. The Taliban in Afghanistan have been mostly cooperative with the polio campaign--in the south of the contry, where their writ is strongest, they even help point out areas missed by vaccine teams--but in 2012 Taliban leaders in Pakistan began banning vaccinations in their areas, condemning the campaign as an American plot. They also started targeting campaign workers for assassination: Since the ban started, 22 people have been killed in attacks on vaccine teams.