“The “loss of a sense of self-identity, delusions of self-identity and experiences of ‘alien control,’ ” observed an elder statesman in the field of microbiome research, are all potential symptoms of mental illness. It made my head spin to think of how many ideas had to be revisited, not least our culturally treasured notions of identity, autonomy, and independence. It is in part this disconcerting feeling that makes the advances in the microbial sciences so exciting.”