[contingo]

The biological term "symbiosis", by itself, in the strict sense, doesn't imply a benefit for both parties, it just describes a persistent and physically close association between two species. Hence parasitism or commensalism are also forms of symbiosis and so are associations which are not obligate, but facultative (both bionts can live without the other, however prevalent the association). "Obligate symbiotic mutualism" is the category for what you're describing. "Endosymbiosis" implies such an integrated relationship that it is at least obligate for the endosymbiont. In this human-mite case there are various claims of benefits and drawbacks to humans from the relationship, and the symbiosis is certainly already obligate for the mites.