Hormones and infectious diseases underpin most aspects of our research. We maintain both field- and laboratory-based research projects that build upon one another in an effort to identify logical direction in research on fundamental questions regarding the physiologies and behaviors of humans, monkeys and apes, with particular focus on explicating the proximate and ultimate causes and consequences of immune-endocrine interactions. We utilize quantitative methods from endocrinology and immunology to test predictions derived from evolutionary biology and aim to understand the interactions between the reproductive endocrine and immune systems, the physiological and ecological determinants of variation in reproductive and immune functions, and ultimately the fitness consequences of this variation. Driven by a dearth of information on hormone-mediated immunity in humans, monkeys and apes relative to other species, we are drawn to the following research questions: 1) how and why do hormones (primarily androgens) mediate immune responses to infectious diseases in males, 2) how and why do hormone levels change in response to immune activation, and 3) how are reproductive and immune functions involved in male life history trade-offs? Our research includes projects on immune-endocrine interactions, honest indicators of immune function, and assessing the energetic costs of immunity.