Recent research has demonstrated that a great deal of variation exists in the abilities of individual birds to resist infection with pathogens. Both genetic as well as environmental conditions shape the immune response of individual birds. By employing standardized immune protocols, we quantify individual immunocompetences in a Mountain White-crowned Sparrow (Zonotrichia leucophrys oriantha) population in Colorado. We then examine how experimentally manipulated food availability, corticosterone levels and prior infection status influence immune function. We review how environmental conditions interact with endogenous factors to shape the outcome of epidemics.