Not everyone believing they have parasites is in some kind of self induced pathological "hole". They are chronically ill; they get no help from doctors; so they go to curezone and other places for help. Right or wrong, that is what people with chronic, apparently unalleviable symptoms tend to do. And "parasites" or "candida" or "lyme" or "low blood sugar" are proposed as causes, even though personally I think that those are not the basic causes; rather, they are symptoms of something deeper.
Hell, for all I know, the fundamental cause could be "evil spirits". And I am not joking about that. Older societies far smarter than ours - with our MDs and their stupid little jars of poisonous pills - knew that emotions/trauma/spiritual illness could be deeply embedded in chronically sick people.