KATHERINE S. WANDER, then a student at University of Washington, Seattle, Washington, received funding in October 2009 to aid research on 'Immunocompetence and the Hygiene Hypothesis,' supervised by Dr. Bettina Shell-Duncan. Extensive research in allergy epidemiology has demonstrated that early exposure to infectious agents is associated with lower risk of allergic disease. An evolutionary perspective suggests that such early exposure may affect not only pathological immune responses to allergens, but also healthy immune responses to pathogens as a developmental adaptation, tailoring immune responses to the local infectious disease ecology. To evaluate this hypothesis, this project evaluated associations between early life infectious disease exposure and: 1) allergic disease; and 2) delayed-type hypersensitivity to Candin (an immune response to pathogen antigen, indicating immunocompetence). Consistent with finding in the US and Europe, large family size was associated with lower risk of diagnosed allergic disease. Consistent with the hypothesized developmental adaptation in immune system development, large family size, hospitalization during infancy with an infectious disease, and BCG vaccination scar were positively associated with immunocompetence (delayed-type hypersensitivity to Candin). These results suggest that not only do early infections discourage the pathological immune responses that result in allergy, they also promote healthy immune responses to pathogens, reflecting adaptive plasticity in immune system development.