Dear Dr. Bob,   We just went to a new pediatrician today and she advised giving the polio vaccine to my 18 month old at this visit do to the fact that we bring him to pools to swim.  She said that polio could be passed through the water if someone was 'shedding' after a visit to a country with active polio cases.  I am concerned about this and had never heard of this before.  How likely is this?  I would have thought that all of the chlorine and other chemicals in...