Parasitism is a relationship in which one organism, know as the parasite , live in or on another organism , know as the host , frome which it deriver nourishment . gernerally , the parasite is much smaller than the host . although the hose is harmed by the interaction , it is gernerally not killed immediately by the parasite , and some host individuals may live a long time and be relative little affected by their parasites . some parasite are much more destructive than other ,however. Newly established parasite/host relationships are are likely to be more destructive than those that have a lone evolutionary history. With a long standing interaction between the parasite and the hose , the two species generally evolve in such a way that they can accommodate one another . it is not in theparasite is best interest to kill its hose . if it does , it must find another . like wise , the hose evolves defenses against the parasite , often reducing the harm done by the parasite to a level the host can tolerate.