Though it seems that the existence of tolerance is mostly adaptive, allowing an adjustment of the immune response to a level appropriate for the given stressor, it comes with important evolutionary disadvantages. Some infectious microbes take advantage of existing mechanisms of tolerance to avoid detection and/or elimination by the host immune system. Induction of regulatory T cells, for instance, has been noted in infections with "Helicobacter pylori", "Listeria monocytogenes", "Brugia malayi", and other worms and parasites. Another important disadvantage of the existence of tolerance may be susceptibility to cancer progression. Treg cells inhibit anti-tumor NK cells. The injection of Treg cells specific for a tumor antigen also can reverse experimentally-mediated tumor rejection based on that same antigen. The prior existence of immune tolerance mechanisms due to selection for its fitness benefits facilitates its utilization in tumor growth.