Good Parasites?

I have already done a parasite cleanse and I am about to do another one. I was telling my friend about it and she is going to medical school to be a doctor and she was telling me about how I should be careful and that there are some parasites that are good for you and that we need. She said there are some that help our digestion and some in your eyelashes we need as well (she went into more detail which I cannot remember off the top of my head right now). I was under the impression that parasites were were not good for you and cause disease. So I figured I would ask some experts here. So whats the deal here?