I wonder, too...if there are so many toxins loose in our environment...
...Aren't parasites likely to be affected by those, too?
...Isn't it likely for a sedentary life-style (more sedentary than our ancestors), to cause deposits/blockages of various kinds to accumulate in organs and tissues?
Also, I think our objective is more to become progressively more immune, in ways so complicated that we can't possibly imagine, so that we can forget about parasites entirely, and never suffer again...I hope.