A mass vaccination against measles has been launched by Medical charity Doctors Without Borders (MSF) in southern Ethiopia hosting Somalis.
The Dolo Ado camps host 118,400 Somali refugees, including 78,000 who arrived this year, and on the first day 3,000 children were vaccinated.
"(Among) our 10 priorities, the first is vaccination campaign for measles," MSF emergency coordinator Guillem Perez told AFP.
The UN refugee agency at the weekend announced a measles outbreak in the camps where the residents are especially vulnerable to communicable diseases due to congestion and immunity weakened by malnutrition.
The vaccination is targeting children under 15.
"Children are the most vulnerable, so they are our first priority," said UN refugee agency spokeswoman Laura Padoan, who added that if the outbreak spreads, adults will also be vaccinated.
Many of the Somali refugees also suffer from malnutrition and aid agencies voiced worry their plight be made worsr by the measles outbreak.
"Malnutrition is very high, so if you mix malnutrition with measles, the scenario is very bad in terms of public health," Perez said.
For Hassan Majero, who fled Somalia to seek refuge in neighbouring Ethiopia, the immunisation came too late. Three of his five children have measles.
"They felt sick and then they had diarrhoea and then pimples appeared," Hassan recounted.
"I am very worried," he added, sitting among his children at the health centre in the camp.
A harsh and prolonged drought in the Horn of Africa has left some 12 million people facing starvation and aid agencies are scrambling to deliver aid and medicine to help victims of the region's worst drought in decades.
Parts of Ethiopia, Djibouti, Kenya and Uganda have been affected by the drought, but war-wracked Somalia has been the worst hit in the region.
Thousands of Somalis have fled to neighbouring Kenya and Ethiopia for help.
Ethiopia this week began relocating Somali refugees to a fourth camp opened last Friday. It will house some 15,000 people.