Ecological immunology provides a broad theoretical perspective on phenotypic plasticity in immunity, that is, changes related to the value of immunity across different situations, including stressful situations. Costs of a maximally efficient immune response may at times outweigh benefits, and some aspects of immunity may be adaptively suppressed. This review provides a basic overview of the tenets of ecological immunology and the energetic costs of immunity and relates them to the literature on stress and immunity. Sickness behavior preserves energy for use by the immune system, acute stress mobilizes "first-line" immune defenders while suppressing more costly responses, and chronic stress may suppress costly responses in order to conserve energy to counteract the resource loss associated with stress. Unexpected relationships between stress "buffers" and immune functions demonstrate phenotypic plasticity related to resource pursuit or preservation. In conclusion, ecological models may aid in understanding the relationship between stress and immunity.