Intestinal worms have figured out a way to keep themselves protected from your body's immune system, by taking control of part of it. The job of flushing out these worms would normally go to your T cells, which attack invaders. But the worm Heligmosomoides polygyrus makes a protein which restrains the production of T cells, thus preventing your immune system from kicking the worms out of your gut.
But there is an unexpected consequence of playing host to these worms. The way they tamp down your immune system's response also suppresses allergic reactions, saving you from the ravages of hayfever. Yay?
Researched published in the Journal of Experimental Biology