Geneva, Reuters—Polio has broken out among young children in northeast Syria, the World Health Organization (WHO) confirmed on Tuesday, and could spread inside and outside of the country, where civil war has led to falling vaccination rates.

Polio, a crippling disease caused by a virus transmitted via contaminated food and water, can spread rapidly among children under five, especially in the kind of unsanitary conditions endured by the displaced in Syria or crowded refugee camps in neighboring countries.

In the Deir Al-Zor province bordering Iraq, 22 children became paralyzed on October 17 and WHO’s regional laboratory in Tunis has isolated the wild polio virus in samples taken from 10 victims. Results on the other 12 are expected within days.

“Out of those 22 being investigated, 10 are now confirmed to be due to polio virus,” Oliver Rosenbauer, spokesman of the WHO polio eradication program, told a news briefing in Geneva.

Most victims are under two years old and are believed never to have been vaccinated or to have received only a single dose of the oral vaccine instead of the three which ensure protection from polio, he said.

Due to the war in Syria, half a million children have not been vaccinated against polio and debilitating diseases such as measles, mumps and rubella, according to the UN Children’s Fund (UNICEF).

It is Syria’s first polio outbreak since 1999, according to WHO, the United Nations health agency.

“Immunizations have started in that area,” Rosenbauer told Reuters, referring to Deir Al-Zor.

The city of Deir Al-Zor is partly controlled by Syrian government forces while the surrounding countryside is in the hands of rebels fighting to remove President Bashar Al-Assad.

Some 65,000 children under five in Deir Al-Zor province are deemed vulnerable, according to the WHO’s latest estimate.

Anthony Lake, executive director of UNICEF, said he had held “businesslike and encouraging” talks with Syrian Prime Minister Wael Al-Halqi and other senior officials. On Tuesday, he called for 500,000 Syrian children to be vaccinated against polio and other diseases.

“With cases of polio now emerging in Syria for the first time since 1999, reaching every child with polio and other vaccinations is not only an urgent and critical priority for Syria, but for the whole world,” Lake said in a statement at the end of his previously unannounced two-day visit to Damascus.

Across Syria there are three million children aged under five.

Young Children Vulnerable

“Before the conflict, which began with peaceful protests in March 2011 and led to a civil war, 91 percent of Syrian children were vaccinated against diseases including polio, but the rate has fallen to approximately 68 percent,” Rosenbauer said.

“So it makes sense that very young kids would get it.”

Polio invades the nervous system and can cause irreversible paralysis within hours.

It is endemic in just three countries—Nigeria, Pakistan and Afghanistan—raising the possibility that foreign fighters have imported the virus into Syria, where Islamist militant groups are part of the splintered array battling Assad’s forces.

“The next step will be to look genetically at these isolated viruses and see where they came from. That should give some clarity on the origin,” Rosenbauer said.

With approximately 4,000 refugees fleeing Syria every day, immunization campaigns for polio and other major childhood diseases—mumps, measles and rubella—are planned in neighboring countries, where there may be gaps in coverage.

“Of course, this is a communicable disease. With population movements it can travel to other areas. So the risk of it spreading across the region is high,” Rosenbauer said.