Within any host population, the effectiveness of the immune response to a given pathogen will vary due to many factors, including previous exposure to related pathogens, genetic heterogeneity, nutrition, age, and immunological consequences of co-infections. Less effective immune responses may lead to increased susceptibility to infection, increased transmission rates arising from higher pathogen loads, or prolonged durations of infection; alternatively they may lead to disseminated disease and rapid host death, and hence lower infectiousness. For any disease-host system, heterogeneous host immunocompetence will cause variations in key epidemiological parameters that are correlated at the level of individual hosts. For example, more susceptible individuals may also have higher rates of transmission, or longer-lasting infections. In this presentation, I will describe the impacts of these correlated traits on the stochastic dynamics of pathogen emergence, with emphasis on acute, directly-transmitted pathogens such as influenza or SARS.