Why might a parasitic worm want to manipulate the human immune system?
…Parasitic worms face daunting transmission problems and must survive for long periods in their vertebrate hosts to achieve reproductive success. They evolved methods of reducing the inflammatory response of their host’s immune system, in part mediated by regulatory T cells, to avoid being killed by it. Regulatory T cells are conspicuous by their absence in autoimmune patients; worm infections appear to restore them to normal levels. Once worms had evolved the ability to persist in their hosts and produce chronic infections, their hosts co-evolved to further reduce the debilitating inflammatory responses elicited by the worms, which had become predictable parts of the environment that could no longer be avoided. Both partners in the host-parasite interaction changed. Then, when modern hygiene eliminated the worms, our immune system was left with inappropriate reactions to the sudden lack of a chronic stimulus.
