David- My understanding is that many healthy people live unaffected by parasites. I believe all humans have some parasites that aren't good. These healthier people are not really bothered by a few bad pathogens. These folks have strong immune systems and very good intestinal flora that can take care of a few intruders.
Someone like myself who has been weakened by many contributing factors has been very susceptible to parasites. To the point that these had kept me housebound for years in my early forties.
It is not that they just appear spontaneously but a sickened person does not have the same ability to fight them off in the first place.
Many here on the parasites forum have many other health issues besides parasites.
Speaking of primitive people. Weston Price claimed that many tribes around the world had perfect teeth and health until they started eating modern food.