Power of suggestion

[Dmitri]

Dear Indologists!

There is a passage in traveling notes ALTAI-HIMALAYA by Nicholas Roerich
(about 1924)

...in the Malabar Hills dark persons may come and because of an unfulfilled
requests will try to touch you, while they say to you:
"Sahib will be sick" or "You will live onlyten days"
If the organism at that moment is fatigued or if the will is weak,
the command is fullfilled and one can remedy this only by a
counter-suggestion.


My questions are:
How much truth there is to this statement?
Have you heard or know first hand of any similar things?
Is there a special name for such "dark persons" or this type of suggestion?

Best regards, Dmitri.