In the broad sense, symbiosis means simply "living together"—the union of two separately evolved organisms into a single functional unit regardless of the positive or negative influence on either species.

Symbiotic relationships fall into three categories. Mutualism describes the condition in which both organisms benefit from the relationship, commensalism exists when one organism benefits and the other is unaffected, and parasitism describes a situation in which one organism benefits while the other suffers. Symbionts may also be classified according to their mode of life. Endosymbionts carry out the relationship within the body of one species, the host. Exosymbionts are attached to the outside of the host in a variety of ways, or are unattached, seeking contact for specific purposes or at particular times. Similarly, symbionts may either be host-specific, co-evolved to one species in particular, or a generalist, able to make use of many potential partners. In...