With only one psychiatrist and thousands of mentally-ill patients, Zanzibar is on the cusp of a mental health crisis. I travelled to the archipelago to meet some of those afflicted by psychotic, neurological and psycho-social disorders. What I found was mass mistreatment and exploitation at the hands of profiteering exorcists and herbalists. It makes for disturbing but compelling listening. Here's the trailer for my programme, which will run in mid-October on the BBC World Service.