WASHINGTON (AP) — We share our bodies with trillions of microbes that are critical to staying healthy, but now scientists are getting a much-needed close look at how those bugs can get out of whack and spur disease.

One lesson: A single test to see what gut bacteria you harbor won't tell much. Research published Wednesday found repeat testing spotted the microbial zoo changing in ways that eventually may help doctors determine who's at risk of preterm birth, inflammatory bowel disease, even diabetes.

At issue is what's called the microbiome, the community of bacteria, viruses and fungi that li...