Though parasites, just like in a garden, do come to clean up dead, dying and deform or diseased tissue (plant or human)
I know in the 70/80's they were still using maggots in surgical incisions at hospitals to eat up rotting tissue after surgeries. They put maggots int he body and sewed you up.
If you have a healthy body it WILL take care of you. But you can not feed it processed foods and expect this of your body. You are what you eat. Any parasites that get past your stomach acid (if it is strong and healthy) would be quarantined (tumor), broken down and carried off (lymph system) unless you have a diseased body. Just like a car needs, not just any but specific gas, oil, antifreeze, air in the tires, etc; your body too needs specific nutrients in specific ratios to function properly. Then none of this or illness is an issue.