Howdy,
   sorry I don't have the original posting for who started this
thread. What I remember is a conversation with someone who had done
agricultural research in India, namely the study of nematodes, and
specifically, the study of hookworms. What he found was that to keep
the incidence of hookworms down in people (which could be absorbed
through the skin, when in contact with infected fecal matter) was the
practice of keeping a 'zone' around each dwelling where no one was to
relieve him or herself. This 'zone' was only violated by the very old,
 the very young, or the very sick.
   Perhaps what you may need to look at are pests or diseases native
to the areas from which the individuals came from.
 
Joanne Reiter