We live in close & constant association with millions of microorganisms, most of which do us no harm (commensals) and some which benefit us (mutualists). Distinguishing how the interactions between host & symbionts differ depending on the nature of the relationship (pathogenic vs non-pathogenic) is currently an active area of research. Using what you've learned, propose some possibilities for how commensal species, in contrast to infectious species, might live with us in peaceful symbiosis. Consider commenting on ways the immune system remains refractory to commensals, compared to pathogens.