JERUSALEM (AP) — The U.N. health agency and partners are launching a campaign starting Sunday to vaccinate 640,000 Palestinian children in Gaza against polio, an ambitious effort amid a devastating war that has destroyed the territory's healthcare system.

The campaign comes after the first polio case was reported in Gaza in 25 years — a 10-month-old boy, now paralyzed in the leg. The World Health Organization says the presence of a paralysis case indicates there could be hundreds more who have been infected but aren’t showing symptoms. Most people who have polio do not experience symptoms, and those who do usually recover in a week or so.

But there is no cure, and when polio causes paralysis it is usually permanent. If the paralysis affects breathing muscles, the disease can be fatal. The vaccination effort will not be easy: Gaza’s roads are largely destroyed, its hospitals badly damaged and its population spread into isolated pockets.

WHO said Thursday that it has reached an agreement with Israel for limited pauses in the fighting to allow for the vaccination campaign to take place. Even so, such a large-scale campaign will pose major difficulties in a territory blanketed in rubble, where 90% of Palestinians are displaced. How long will it take? The three-day vaccination campaign in central Gaza will begin Sunday, during a “humanitarian pause” lasting from 6 a.

m. until 3 p.m.

, and another day can be added if needed, said Dr. Rik Peeperkorn, WHO’s representative.