Causes
A bacterial infection that thrives when the immune defences drop: incorrect temperature or humidity, stress, poor hygiene, mouth injuries, poor nutrition.
Diagnosis
Examination of the mouth by an exotics vet, sometimes with a culture.
Treatment
Cleaning of the mouth, antibiotics, and urgent correction of the terrarium conditions. Advanced cases can affect the bone.
Prevention
Keep the terrarium in optimal conditions (temperature, humidity, hygiene) for the species, handle carefully, provide good nutrition, and reduce stress.

