>>They are programmed to live under the radar. Obviously something has to trigger these parasites to "awaken" them
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Old Wives tales, the immune system gives up at some point.
I think Ascaris is one of the best examples of this. In spring some people get allergies because certain pollen from weeds is a remedy against Ascaris.
So when their energy slightly diminishes your immune system gives it yet another shot to kill them..without luck.