This is not exactly true. When I was studying this stuff in school I can remember various accounts of shamans, bokors and houngans, witches etc who were basically at the mercy of the spirits(to them anyway they were)..they basically HAD to shamanize or they would die(again in they're words). They had no say in any of it..they were like gang raped by the spirits, pulled into that life weather they liked it or not. They do the work becasue they must. The shaman might very well live out they're entire life with the spirits that once invaded them..the only difference between them and the patient is that the shaman has made deals with these spirits and now comes to them as familiar aquaintances and "coworkers".