Parasites have evolved from free-living organisms which were preadapted to parasitism. Parasites have developed a series of strategies that allow them to survive in equilibrium with their hosts. This in turn leads to tolerance as a result of an adaptive process between both members of the parasitic association. Parasites compete with their hosts for their nutrients and materials and are able to destroy or transform their tissues. Likewise, parasites have developed several mechanisms that allow them to evade the immune system of their hosts. They avoid destruction by complement, thus surviving immune attack or the intracellular killing. Other parasites cover themselves by mimicry or antigenic variation. They hide generating cysts or nodules, colonizing the gut, invading immune privileged sites or by means of rapidly internalizing within host cells. They are able to modulate the immune system by immunosuppression, production of homologous host cytokines or huge amounts of soluble antigens or mitogens, thus affecting the balance of cytokines or antigen presentation.