Memorial Sloan Kettering Cancer Center in New York has also started a proactive stool-banking study. Most of the subjects are patients with leukemia. Before stem cell transplants, patients receive antibiotics and chemotherapy, often wiping out their microbiota.

Dr. Eric Pamer, a physician and scientist at Memorial Sloan Kettering, has discovered that the diversity of the microbiota just after the stem cell transplant predicts well-being and survival. After excluding death from leukemia recurrence, those with the least diverse microbiomes after surgery were five times less likely to remain alive three years later, when compared with those with the most diverse.

“We often wipe out the flora,” Dr. Pamer told me. “It should become a routine part of practice to restore the flora.” It’s “very reasonable,” he added, to apply this principle to everyone who receives broad-spectrum antibiotics, not just leukemia patients, although the idea needs testing.

In mice, simply caging an antibiotic-treated animal with a nontreated one will restore its microbiota. Mice are “coprophagic,” though. They eat one another’s feces. People don’t — or so we like to think. In reality, for much of our evolution, we shared more of our microbes. This pre-sanitary past was disease-ridden, but it may also have enabled acquisition of health-promoting microbes. One recent study in Papua New Guinea found that, in an environment with fewer sanitary amenities, people’s microbes seemed to recover naturally from antibiotics.

Shouldn’t we have a more precise microbiota-recovery protocol? I called up Mark Smith, founder of a nonprofit stool-banking organization called OpenBiome. Years ago, as a doctoral student at M.I.T., he’d watched a friend spend a year and a half sickened by C. difficile. Antibiotics failed to help and, at wits’ end, the friend finally gave himself a stool transplant, in his own apartment, with stool donated from his roommate. It worked.

Dr. Smith couldn’t understand why this highly effective treatment for a widely recognized scourge was relegated to D.I.Y. Lack of banked, screened fecal material, he discovered, was one hurdle. So he started OpenBiome, which screens donors and banks stool for use by medical professionals. Business is booming, he told me. This month, he started a pilot self-banking program called “PersonalBiome.” One complication: If he stores your stool, you can generally withdraw it only to treat C. difficile, not for preventive “reconstitution.” That’s because stool is regulated as a drug and not, as with embryos or blood, a tissue, which makes its use more complex.