“No man is an island entire of itself,” wrote English poet John Donne, referring to the interconnectedness of the human species, and our societal dependence on each other. Almost 400 years later, ongoing research into microbiomes shows that interconnectedness is bigger and also much, much smaller than we once imagined. And it indicates that the health of all life on this planet — including our own — is predicated on communities of minuscule microorganisms, working in tandem, that proliferate everywhere around and in us.