The Taliban have suspended polio vaccination campaigns in Afghanistan, the U.N. said Monday. It’s a devastating setback for polio eradication, since the virus is one of the world’s most infectious and any unvaccinated groups of children where the virus is spreading could undo years of progress.

Afghanistan is one of two countries in which the spread of the potentially fatal, paralyzing disease has never been stopped. No reason was given for the suspension.

On Monday, the AP news agency reported suspending the polio vaccination campaign in Afghanistan. Official reasons were not provided, but the opposition claims it is about limiting women's participation in vaccination efforts.