How diseases affect symbiotic communities

Examples of species living in symbiotic communities are well known among biologists. In this study we extend our previous analysis of predator-prey or competing models subject to a disease spreading among one of the species to populations mutually benefiting from interactions. We assume that one of the species is subject to a disease which cannot cross the species barrier and investigate the behavior of such a community. Our findings indicate that there are parameter ranges for which an interesting phenomenon occurs. Namely, the equilibrium shifts to a {\em higher} value. That is, the disease seems to have a positive effect on the environment as also field experiments show.