Pathocoenoses ancient and modern

Problems concerning the concept of biocoenosis in ecology (the antecedent of the pathocoenosis concept) are discussed first of all. Six main problems are identified: the problem of emergent properties of ecological communities; the problem of ambiguity; the problem of heterogeneity; the boundary problem; the problem of retrospective differential diagnosis; and the problem of explaining change over time. The rest of the paper gives illustrations of these problems in relation to human pathogens drawn mainly from the interactions of malaria with other diseases, particularly but not exclusively in the Mediterranean world, from antiquity through to modern times.