Upsets in some of the delicate balances necessary to maintain commensalism between two organisms can result in benefit for one organism in the relationship at the expense of the other, i . A host may need to increase food intake to compensate for the effects of a parasite. Cells and proteins will be directed toward maintaining an immune response against a parasite, expending resources that could be needed to fight a more serious pathogen. Increased needs for nutrients may force the host to modify its behavior patterns in order to obtain more food, thus exposing the host to other potential threats. Parasitism over extended periods of time can be an expensive circumstance for the host.
Parasites can compete with the host for vital substances .
Parasites can put pressure on host energy utilization and reserves . Parasites can cause pain . Parasites can provoke adverse changes in host behavior . Parasites that feed on skin or live most of their lives in association with the integument of the host, thus producing an infestation , are known as ectoparasites . Parasites that dwell primarily in the internal tissues of the host, thus producing an infection , are known as endoparasites .