Pathogens are just one component of complex biological systems.
What is perceived as a negative effect or positive effect of disease depends on the context in which pathogens operate. In native environments, pathogens are an integral and often fundamental force in ecosystem dynamics and renewal, contributing to ecosystem biodiversity and long term stability and viability. In production contexts or in urban environments, both cases of highly simplified, and therefore unstable, ecosystems, diseases can have destructive consequences. Very often, diseases predispose trees to other negative forces, like insect pest attacks, which contribute to the eventual demise of the host tree. Other times, as with other plant-pathogen systems, disease may actually increase resistance to the same or other diseases or to insect attack.