If you tried to determine a woman’s health by analyzing her vaginal microbes, the results would be hard to interpret and might be outdated by the time they arrived.

This befuddling complexity is not confined to the vagina. Earlier this year, Patrick D. Schloss at the University of Michigan analyzed microbes from 18 different body parts on 300 volunteers. They were all healthy, with nary a dental cavity among them. And yet, Dr. Schloss found that their microbes varied greatly, and flipped between different states, for as yet inexplicable reasons.

The dynamic nature of the microbiome partly explains the enthusiasm that surrounds it. If scientists identify changes in the human genome that increase the risk of disease, it is hard to rewrite those genes or to find drugs that target them. But the microbiome could theoretically be altered through probiotics, fecal transplants or other means. It is, as some researchers say, the only “organ” that can be replaced without surgery.

But how can you tell when it needs replacing? A bloom of C. difficile is an obvious problem, but most other communities are not so easily classified. The microbiome is a teeming collection of thousands of species, all constantly competing with one another, negotiating with their host, evolving, changing. While your genome is the same as it was last year, your microbiome has shifted since your last meal or sunrise.

We need to start thinking about it as an ecosystem, like a rain forest or grassland, with all the complexities that entails. And just as the gorillas and leopards of African forests differ from the wolves and moose of American ones, so, too, do microbiomes vary around the world.

Take the Hadza. Their microbial roll call is longer than a Western one, with both omissions and additions. They are the only adult humans thus far sequenced who are devoid of Bifidobacteria — a supposedly “healthy” group that accounts for up to 10 percent of the microbes in Western guts. But they do carry unexpectedly high levels of Treponema, a group that includes the cause of syphilis.

Is this menagerie worse than a Western one? Better? I suspect the answer is neither. It is simply theirs. It is adapted to the food they eat, the dirt they walk upon, the parasites that plague them. Our lifestyles are very different, and our microbes have probably adapted accordingly. Generations of bacteria can be measured in minutes; our genomes have had little time to adapt to modern life, but our microbiomes have had plenty.