In this chapter, I consider several metaphysical questions in the philosophy of public health. These can be split into two broad categories, though often the demarcation in the literature between the two is subtle or entirely missing. On the one hand, there are ontological questions. These concern the nature of the entities or processes involved. On the other, there are questions pertaining largely to conceptual analysis; that is, what particular concepts mean, given the context. Both are important, and both will be addressed. I examine models of disease causation, considering both the monocausal and multifactorial schools of thought. Whilst this is largely a project in conceptual analysis, I also examine the ontological implications, ending the section with the dispositional model of disease etiology in individuals, and applying this to populations. I end the chapter by discussing the ontology of epidemics, suggesting that epidemic-status, like health-status, should be considered an emergent property of populations, and developing this idea in the context of my capacities approach to measuring population health.