Ecoimmunology is an interdisciplinary field combining aspects of immunology with ecology, biology, physiology, and evolution. The field of ecoimmunology, while young, seeks to give an ultimate perspective for proximate mechanisms of immunology.
Classical, or mainstream, immunology works hard to control variation (inbred/domestic model organisms, parasite-free environments, etc.) and asks questions about mechanisms and functionality of the immune system using a reductionist method. Comparative immunology investigates the major changes of the immune system among taxa. While ecoimmunology originated from these fields, it is distinguished by its focus to describe and explain natural variation in immune functions, and, more specifically, why and how biotic and abiotic factors contribute to variation in immunity in animals. Study of the trade-offs between immunity and other physiological mechanisms are a central study topic within the field, but have been expanded to include roles in species and individual variation, sex, social aspects, and mating system differences, and progress is also being made to develop methods to explore this variation. Many studies involve in vivo laboratory experiments, but there have been recent calls for immunologists to study immune variation more in wild animals in particular. Multiple institutes engage in ecoimmunological research, such as the Center for Immunity, Infection and Evolution at the University of Edinburgh and the Max Planck Institute for Immunoecology and Migration. The US National Science Foundation has funded a Research Coordination Network) to bring methodological and conceptual unity to the field of ecoimmunology.
One of the field’s most seminal papers, by Folstad and Karter, was a response to Hamilton and Zuk’s famous paper on the handicap hypothesis for sexually selected traits. Folstad and Karter proposed the immunocompetence handicap hypothesis, whereby testosterone acts as a mediator of immunosuppression and thus keeps sexually-selected traits honest. Although there is only moderate observational or experimental evidence supporting this claim up until now, the paper itself was one of the first links to be made suggesting a cost to immunity requiring trade-offs between it and other physiological processes. In 1996, a foundational paper for the field invoked trade-offs, the allocation of limited resources among competing, costly physiological functions, as a prime cause of variation in immunity. Evidence for these putative trade-offs has often proven to be elusive 
More recently, ecoimmunology has been the theme of two special issues in peer-reviewed journals, one in Philosophical Transactions of the Royal Society B and the other in Functional Ecology (see External links).
- Sheldon, BC; Verhulst, S (1996). "Ecological immunology: costly parasite defences and trade-offs in evolutionary ecology". Trends in Ecology and Evolution. 11 (8): 317–21. doi:10.1016/0169-5347(96)10039-2. PMID 21237861.
- Martin, LB; Hawley, DM; Ardia, DR (2011). "An introduction to ecological immunology". Functional Ecology. 25: 1–4. doi:10.1111/j.1365-2435.2010.01820.x.
- Pedersen, ABP; Babayan, S (2011). "Wild Immunology". Molecular Ecology. 20 (5): 872–80. doi:10.1111/j.1365-294X.2010.04938.x. PMID 21324009.
- Folstad, I; Karter, AJ (1992). "Parasites, bright males, and the immunocompetence handicap". American Naturalist. 139 (3): 603–22. doi:10.1086/285346. JSTOR 2462500.
- Hamilton, WD; Zuk, M (1982). "Heritable true fitness and bright birds: A role for parasites?". Science. 218 (4570): 384–87. Bibcode:1982Sci...218..384H. doi:10.1126/science.7123238. PMID 7123238.
- Downs, CJ; Schutz, H; Meek, TH; Dlugosz, EM; Acosta, W; de Wolski, KS; Malisch, JH; Hayes, JP; Garland, T, Jr. (2012). "Within-lifetime trade-offs but evolutionary freedom for hormonal and immunological traits: evidence from mice bred for high voluntary exercise" (PDF). Journal of Experimental Biology. 215 (Pt 10): 1651–1661. doi:10.1242/jeb.066167. PMID 22539732.
- NSF Research Collaborative Network: Refining and Diversifying Ecological Immunology
- Wild Immunology, University of Edinburgh
- Ecological Immunology Theme Issue of Philosophical Transactions of Royal Society B Table of Contents
- Special issue: Ecological Immunology, Functional Ecology Table of Contents