Two children have been diagnosed with polio paralysis in Nigeria. It’s a major setback to the international effort to eradicate the disease, which health authorities thought they had sequestered into only Pakistan and Afghanistan.

The news was announced by the World Health Organization and the Centers for Disease Control and Prevention Thursday, a day that would otherwise have marked the two-year anniversary of Africa’s last case of polio. If Thursday had come and gone without any cases being discovered, that would have triggered a 12-month countdown to the entire continent’s being certified free of polio, a crucial step in the almost 30-year battle to wipe the disease from the world.

“This is a setback, definitely, these two cases that have