We all have parasites to some degree. When they're kept in check they don't cause problems. When they start multiplicating and releasing their poisons, then symptoms start to appear.
Natway, you have really no knowledge regarding parasite infestations. They're real, and most medical tests come back negative. In my experience, I was passing dozens of worms and parasites as long as 8 inches daily, and my tests always came back negative. Doctors are just useless.
When someone is seriously sick, you can assume parasites are part of the equation. Parasites thrive in weak digestive systems.