Last year the country was declared polio-free. Now there are three new cases. And Boko Haram militants are the biggest obstacle to getting rid of the virus.
155 countries and territories are switching to a different polio vaccine, in what health officials say is the next step to completely eradicating the disease worldwide.
What will be the big stories of 2016? Four experts offer their best guesses. They worry about Zika virus and malaria. But not all their predictions are downers.