What if it is the other way around :
The human body is not equipped to fight all kinds of parasitic worms and some parasitic worms in a human body will increase sensivities to some foods, decrease absorbtion, decrease reserves, stress the body, decrease circulation, irritate, inflame some parts of the body and therefore alter the functionning of some organs, increase toxins and waste load.
I eat mostly beneficial foods, i drink enough, i sleep enough, i exercise regularly, i am fitter stronger faster more agile more endurant than most people that i have met, but i am infected probably since i am a teenager... I bet that all humans are infected by one or several kinds of parasitic worms, they are just not aware of them...
Read some books about parasitology, you will see that there is no such thing as being immune to all kinds of parasitic worms.