i think one has to distinguish between biological and chemical
contaminants.
with biological contaminants, you can gain resistance with
strong immune system due to frequent exposures
your body develops anti-bodies.....................
with chemical contaminants, you drink abit with frequency it
will accumulate in your system
consider drinking arsenic, you drink minute amounts, it doesn't
kill you, you drink for years
then one fine day, you decide that you have build up resistance
to it, you drink in large does
you die.
does it mean with increased frequency of exposure
you will gain resistance to it?
Not to forget some of these comtaminants may be carcinogenic.