The most comprehensive survey of how we share our microbiomes suggests a new way of thinking about diseases that aren’t usually considered contagious.

OUR BODIES CONSIST of about 30 trillion human cells, but they also host about 39 trillion microbial cells. These teeming communities of bacteria, viruses, protozoa, and fungi in our guts, in our mouths, on our skin, and elsewhere—collectively called the human microbiome—don’t only consist of freeloaders and lurking pathogens. Instead, as scientists increasingly appreciate, these microbes form ecosystems essential to our health. A growing body of research aims to understand how disruptions of these delicate systems can rob us of nutrients we need, interfere with the digestion of our food, and possibly trigger afflictions of our bodies and minds.

But we still know so little about our microbiome that we are just starting to answer a much more fundamental question: Where do these microbes come from? Can they spread from other people like a cold virus or a stomach bug?

Now, the largest and most comprehensive analysis of human microbiome transmission has provided some important clues. Research led by genomicists at the University of Trento in Italy have found hints that microbiome organisms hop extensively between people, especially among those who spend a lot of time together. The findings, published in January in Nature, fill important gaps in our understanding of how people assemble their microbiomes and reformulate them throughout their lives.

Other scientists have applauded the study. Jose Clemente Litran, an associate professor of genetics and genomic sciences at the Icahn School of Medicine at Mount Sinai, hailed the work as “outstanding” and said it provided the first clear measure of how much sharing to expect among family members or those who live together.