Is it ensure, insure, or assure?
parasitism par·a·sit·ism (pār'ə-sĭ-tĭz'əm, -sī-)
A symbiotic relationship in which one species, the parasite, benefits at the expense of the other, the host.
A symbiotic relationship in which one organism (the parasite) benefits and the other (the host) is generally harmed. Parasites derive nutrition from their host and may also gain other benefits such as shelter and a habitat in which to grow and reproduce. See more at parasite. Compare amensalism, commensalism, mutualism.