ABSTRACT
A wide variety of environmental factors influence symbiotic infections and diseases in fishes. Estuarine and marine habitats are less likely to offer such confinement, but additional factors in such habitats can complicate both infections and infectious diseases, especially when they constitute stresses that stimulate shift of an equilibrium from infection to disease condition. The numerous factors that influence infections interrelate in such a way that the observed dynamic effects produce a complex web of interactions, in many cases nearly impossible to delineate or to observe duplicated in a natural setting. The prevalence and intensity of an infection as well as other aspects may depend on whether the parasite is external or internal, whether it has a direct life cycle or an indirect one, and whether it can have an adverse effect on its fish host. Fishes exhibit an immunological response to some protozoans, helminths, and other metazoans, in addition to microbial agents.
