Studies comparing incidence of foodborne illness among different population subgroups are complicated by biases inherent in routinely available data. Even when we account for these biases, our understanding of the epidemiology of foodborne illness is hampered by a lack of empirical evidence on how social and cultural factors influence risk, and how this risk is modified by the acquisition of immunity. An epidemiology of foodborne illness must include the epidemiology of food and the complex pathways by which it results in illness, as well as an understanding of the acquisition and prevalence of immunity to foodborne pathogens. Only then can we correctly interpret studies of incidence and risk factors, and their implications for control of foodborne illness.