Yes. Evolutionarily, the adaptive immune system itself is the result of viral integration. Though not a commensal relationship, priming the immune system for

Yes. Evolutionarily, the adaptive immune system itself is the result of viral integration. Though not a commensal relationship, priming the immune system for healthy function, principally for stopping cancer, see high incidence of leukaemia in children not exposed to pathogens (this might be pathogens more generally, it been a while since I took immunology but the immune system, treats cancer much more like viruses than it does bacteria (n.b. broad overgeneralisation)). See also the use of phage to attack bacterial infections.