Reports on the role of hydrological conditions in the functioning of parasitic systems are analyzed. The authors point out that a range of issues including different aspects of combination phenomena is discussed there. They suggest that a concept of combination shall be replaced by a concept of association of parasitic systems and their relevant natural foci of zoonoses, which is of a larger scope. The fundamentals of the formation and functioning of associated parasitic systems and natural foci of zoonoses are discussed. These include the hydrological conditions of floodplain-river regions, the multiparasitic capacity of hosts and vectors, the multihost pattern of causative agents, and a diversity of mechanisms of their transmission. The transmission mechanism without which movement of causative agents and the existence of parasitic systems and foci are impossible is shown to be a global combining onset of parasitic systems. The hydrological conditions of floodplain-river regions act as the universal mechanism of transmission of pathogens. The dynamics of the mechanism determines the functioning of all other mechanisms of transmission.