Health experts fear a resurgence of polio in parts of the world.
Despite a quarter-century-long vaccination programme, the infectious disease is still endemic in Nigeria, Afghanistan and Pakistan, and experts fear it could make a comeback in countries riven by conflict.
Outbreaks in Syria and Somalia, where vaccination programmes have been disrupted, could hamper efforts to rid the world of the curable disease.
BBC News outlines the struggle against polio – in 60 seconds.
Video produced by Adam Blenford and Michael Hirst