Children are said to be at greater risk of developing visceral leishmaniasis (VL) when they are younger and more malnourished. If malnutrition really is associated with VL, this potentially fatal and visible disease may be a general indicator of community health among the rural and suburban poor. Previous conclusions reached about the roles of malnutrition and age in VL epidemiology are questionable because they may have been confounded by transmission rate, because they have not been able to distinguish between different mechanisms of acquiring immunity, and because empirical observations have not been compared with theoretical expectations. Here we offer a framework with which to investigate these questions quantitatively, and do so with published data from endemic areas of Brazil. We conclude that children are indeed more susceptible to VL when they are younger and more malnourished, but it remains unclear whether the immunity to VL acquired with age is always acquired as a result of infection. The significance for leishmaniasis control, and for the control of other diseases associated with malnutrition, will depend on underlying mechanisms, which are not yet understood.