The human “microbiome”—the trillions of bacteria, yeasts, and other microscopic creatures that live inside a human body—has been one of the major science stories in recent years. It seems that barely a week goes by that we don’t learn something new about the relationship between the human body and the vast population of microbes that call it home.

But a study in the January issue of PLoS ONE suggests that the colonization may be more complete than we’ve imagined.