Alarmed by the spread of polio to several fragile countries, the World Health Organization declared a global health emergency on Monday for only the second time since regulations permitting it to do so were adopted in 2007.

Just two years ago — after a 25-year campaign that vaccinated billions of children — the paralyzing virus was near eradication; now health officials say that goal could evaporate if swift action is not taken.

Pakistan, Syria and Cameroon have recently allowed the virus to spread — to Afghanistan, Iraq and Equatorial Guinea, respectively — and should take extraordinary measures to stop it, the health organization said.