Pathogen coexistence induced by density-dependent host mortality.

A model of two competing infectious diseases with complete cross-protection sharing one host allows for coexistence, provided that the host population is subject to strong density regulation. The phenomenon is caused by the different ways in which host density affects transmission rate and transmission period. The analysis suggest that disease coexistence is most likely when the two diseases differ significantly in virulence and transmission and that the evolutionary stability of the two-disease association depends critically on details in the functional relationship between virulence and transmission.