Parasites used to be ubiquitous before we had medication to kill them. There's even a (not very well supported) theory that these parasites helped with allergies by moderating immune system. They releasing chemicals to lower immune activity in order to protect themselves, so the idea that we had these for thousands of years and basically are made to have them is intriguing. It's called "helminthic therapy" and it's considered alternative medicine but there is some academic interest. Results in clinical trials have been mixed. Perhaps the future is just synthetic hookworm proteins that regulate your immune system as our ancestors once had.