Not sure about importance of that compared to larger parasites. One famous ethical problems in science was that people were given infection of... I think it was pig roundworm in stomach. It cannot procreate in human. But the people lost their allergies. So now the ethical question was, do people participating experiment get to keep the worm that is beneficial to them. Anyway, one hypothesis is that allergies are due to lack of parasites when we have a system to fight against them.