It's to do with bacteria basically forces the immune system to work.
There's a real gross treatment for one intestinal superbug which is basically a shit transplant.
From a healthy patient you extract poo. You blend it to make it liquid. You then insert into sick person's arse. All the way-like colonic irrigation in reverse.
24 hours later (so you use a bung?) you let them shit for Britain.
Bug is cured. The flora from the other person's gut sorts the issue out.
The immune system is also implicated in a lot of instinctive responses including falling in "love"

. Look it up on newscientist

.