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About
Snippet
Perhaps
the
most
serious
way
in
which
parasites
that
are
usually
harmless
may
become
of
great
importance
is
illustrated
by
their
introduction
into
new
regions
or,
as
is
more
often
the
case,
by
the
introduction
of
new
hosts
into
the
region
where
the
parasites
are
found.
Under
normal
conditions
the
animals
of
a
given
region
are
usually
immune
to
the
parasites
of
the
same
region.
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