Hi Grizz,
I thought this quote from Dr. John Tilden, M.D., might be of interest [in answer to your last question]: "Normal persons are deadly to all germs and parasites peculiar to the human habitat." So yes, I am of the view that "a strong immune system" does [in fact] "kill off " all parasites. The challenge for most of us, of course, is being healthy enough to have that strong of an immune system!
All just "grist for your mill," Grizz.
Best wishes,
David