In general biological terms, the type of association between two different organisms can be classified as parasitic, where one benefits at the expense of the other, or symbiotic (mutualistic), where both benefit. There is an intermediate category called commensalism, where only one organism derives benefit, living near the other organism or on its surface without doing any damage. It is often difficult to use this category with confidence, because an apparently commensal association often proves on closer examination to be really parasitic or symbiotic, or it may at times become parasitic or symbiotic
