Symbionts and pathogens: what is the difference?

The ecological relationships that organisms establish with others can be considered as broad and diverse as the forms of life that inhabit and interact in our planet. Those interactions can be considered as a continuum spectrum, ranging from beneficial to detrimental outcomes. However, this picture has revealed as more complex and dynamic than previously thought, involving not only factors that affect the two or more members that interact, but also external forces, with chance playing a crucial role in this interplay. Thus, defining a particular symbiont as mutualist or pathogen in an exclusive way, based on simple rules of classification is increasingly challenging if not unfeasible, since new methodologies are providing more evidences that depict exceptions, reversions and transitions within either side of this continuum, especially evident at early stages of symbiotic associations. This imposes a wider and more dynamic view of a complex landscape of interactions.