Luckily you don't need to be infected by parasites to have a good immune system. What seems to work best is exposure to bacteria and viruses and fungi and parasites that don't infect humans. Things associated with other plants and animals. The immune system learns better from the things that it is effective at protecting you from than the things that outsmart it. So let's not bring back the human parasites.

> Let's bring back the worms I guess?

“When Jasper Lawrence learned that hookworms might lessen the effects of his allergies, he set out on a unique mission: To travel to West Africa and purposefully become infected with the parasite” [1].

[1] https://www.thisamericanlife.org/404/enemy-camp-2010/act-thr...

The worms can cause issues. I'd rather no worms.