The cases in Syria are mostly among children who are age two or younger. In this conflict-ridden country, it has been difficult to reach all children for immunization. No one can say how the disease reached Syria, but what the outbreak shows, says Rosenbauer, is "that this is an epidemic-prone disease.”

There is worry that the highly infectious disease will spread farther, as people move in and out of the area to other parts of the country and across borders to refugee camps and nearby countries. “The whole region is now at risk,” Rosenbauer says.

To prevent that, response to the outbreak has already begun. Last month, shortly before the polio cases were confirmed, Syria launched a previously planned vaccination program to