Annelids are soft bodied, segmented worms that inhabit terrestrial, freshwater, and marine habitats. There is no body of research on annelid diseases under laboratory or field conditions. This chapter explores the immunocompetence of annelids, host-parasite interactions, and environmental pollutants that induce fitness costs. The inflicted pathology ranges from minor metabolic changes to major tissue damage resulting in mortality. Minor changes can compromise survival through reducing resilience to stress or impairing mobility. Annelid health research is urgently needed because they are an important food source, which can lead to the trophic transfer of pollutants, and spread of parasitic diseases in wildlife, pets, fowl, swine, fish, and shrimp farming. The absence of annelid health research highlights two completely overlooked risks in environmental management: the assumption that earthworm abundance is indicative of ecosystem health, and the unregulated transport of potentially diseased individuals and vectors around globe through the bait industry.