People who become shamans are believed to be "chosen" by gods or spirits through a spiritual experience known as shinbyeong ("divine or shaman illness"), a form of ecstasy, which entails the possession from a god and a "self-loss".

This state is said to manifest in symptoms of physical pain and psychosis.

Believers assert that the physical and mental symptoms are not subject to medical treatment, but are healed only when the possessed accepts a full communion with the spirit. The illness is characterised by a loss of appetite, insomnia, visual and auditory hallucinations.

The possessed then undergoes the naerim-gut, a ritual which serves both to heal the sickness and to formally establish the person as a shaman.
- from Wikipedia