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Strictly speaking, a ‘symbiont’ is any organism involved in a symbiotic relationship. However, most scientists tend to restrict the term to an organism that lives within or upon another organism and provides it with some form of benefit – usually nutritional. The association is therefore referred to as a host: symbiont relationship and most symbionts are microorganisms such as bacteria, algae, or protozoa. Where the symbiont occurs within the body of its host, it is referred to as an endosymbiont, whilst those attached to the outside are referred to as ectosymbionts. There are two types of endosymbiont: primary endosymbionts (or p‐endosymbionts) and secondary endosymbionts. Primary endosymbionts form obligate relationships with their host and are the product of many millions of years of co‐evolution. They are usually contained within specialised cells and are transferred vertically from mother to offspring. Consequently, they undergo co‐speciation with their host and form very close host‐specific relationships. By contrast, secondary endosymbionts probably represent more recent host: symbiont associations. In the case of insects, these symbionts live within the haemolymph (blood) rather than specialised cells or organs. Secondary endosymbionts tend to be transmitted horizontally and therefore do not show a close host: symbiont relationship. Horizontal transmission occurs when a symbiont (or parasite) is transmitted from one host to another that is not necessarily related to it.

It is uncertain how endosymbionts begin their association with their hosts, but some authors suggest that they arise from pathogens that attenuated over time. The suggestion that a parasite–host relationship tends to start off acrimoniously and then mellows with time is widespread in the literature, and whilst this may sometimes occur it is not a foregone conclusion.