This is the belief that ghosts can cause illness and death. Many tribal societies and those that follow and animistic approach believe there is a time period, usually a few days where the soul remains close to its body after death.
These few days are dangerous to the living because the ghost is lonely and seeks the living for company. Children, because their souls are weaker, and haven’t attached to the body as firmly, are at higher risk. Ghost sickness is part of the reasons behind so many death and funeral customs.
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Mari Wells 
An interesting try at explaining contagion in eras when the causes of disease were not well understood.
Interesting.