Starvation in northern Nigeria’s Borno State is so bad that a whole slice of the population — children under 5 — appears to have died, aid agencies say.

As the Nigerian army has driven the terrorist group Boko Haram out of the area, about two million people have been displaced. Many are living in more than 100 refugee camps.

Doctors Without Borders, which has been in Borno State since 2014, reported in November that it was seeing hardly any children under age 5 at its clinics, hospitals and feeding centers.

“There are almost always small children buzzing around the camps,” Dr. Joanne Liu, the agency’s president, and Dr. Natalie Roberts, an emergency operations manager, wrote then.