A 25-year, US$10-billion global effort has taken the number of polio cases from hundreds of thousands per year to just hundreds, but it is now struggling to stamp the virus out of its final strongholds in Pakistan, Afghanistan and Nigeria, where transmission has never been interrupted. Of these, Nigeria was the only one to see an increase in cases from 2011 to 2012, and public-health experts worry that the virus's recalcitrance here will prevent global eradication, and eventually lead to a wider resurgence of the disease.