Culture-bound syndrome (CBS) is where "certain behavioral, affective and cognitive manifestations only seen in specific cultures."
Basically it means there are diseases that really only exist so long as you believe they do and that you can catch them.
It isn't purely psychological, and often those who believe they have these diseases actually show physical symptoms.
Countries all over the world have these...here are a few of some we find interesting:
Koro is a psychological disorder characterized by "delusions of penis shrinkage and retraction into the body, accompanied by panic and fear of dying."
Saora Disorder is where people "cry and laugh at inappropriate times, have memory loss, pass out, and claim to experience the sensation of being repeatedly bitten by ants when no ants are present."
Ghost sickness is where some Native American tribes believe the dead or dying cause 'general weakness, loss of appetite, a feeling of suffocation, recurring nightmares, and a pervasive feeling of terror."