Children carry the body of a friend who was killed by shelling during heavy fighting between the Free Syrian Army and the forces of Syrian President Bashar Assad in Damascus earlier this year. Momahed Dimashkia/Reuters/Landov hide caption

itoggle caption Momahed Dimashkia/Reuters/Landov

Refugees warm their hands at a refugee camp in Harmanli, Bulgaria, on Nov. 27. More Syrians are turning up in Europe. Many are trying to get to northern Europe, believing that is the best place to start a new life. Nikolay Doychinov/AFP/Getty Images hide caption

itoggle caption Nikolay Doychinov/AFP/Getty Images

Syrian boys line up to get the polio vaccine at a refugee camp in Sidon, Lebanon, on Nov. 7. The Lebanese government plans to vaccinate all kids under age 5 for the virus, including Syrian refugees. Mohammad Zaatari/AP hide caption

itoggle caption Mohammad Zaatari/AP

A health worker administers polio vaccine as part of a UNICEF-supported campaign in Damascus. Aid agencies sometimes have to negotiate with rebel groups and work in dangerous situations to immunize vulnerable children. Omar Sanadiki/AP hide caption

itoggle caption Omar Sanadiki/AP