Though stool banks in the United States said they have a sufficient supply of stool samples to meet the current demand, Canadian researchers said they have fecal transplant programs in only a few cities and anticipate the need to expand stool banks in order to provide services to patients in more remote areas and other parts of the country.

“We are always actively looking for more and more donors,” said Dr. Karen Wong, a co-author on the study. The stool bank at the University of Alberta often runs low, she said, and “sometimes I have to call the donor when we want to do a procedure and ask them to make a donation the day before, so we have the stool ready.”

Becoming a stool donor isn’t easy. Volunteers who apply to be stool donors at OpenBiome, a nonprofit stool bank based in Boston, go through an intensive screening process. First, they fill out an online health questionnaire. Then, based on the answers, they may or may not be invited in for an hourlong clinical interview, which includes questions about their current health status, their health history and their family’s health history. If they pass the interview, their blood and stool undergo extensive testing to screen for pathogens and infectious diseases.

The vast majority of applicants are rejected, said Carolyn Edelstein, OpenBiome’s executive director. “Only about 3 percent make it through our screening process,” she said. “We like to joke with our donors that it’s easier to get into Harvard or M.I.T. than be a stool donor.”

Anyone with allergies or asthma, a family history of colon cancer, autoimmune diseases or infectious diseases, or a high body mass index is usually disqualified, Ms. Edelstein said. “If you traveled to certain regions of the world or recently got a tattoo or recently changed sexual partners, that might exclude you or put you on hold for a few months,” she said. “We count on people to be honest.”

Gabriel Taylor, 22, who just graduated from Tufts University, was one of those rejected. Though he was invited in for an interview in 2016 after filling out the online questionnaire, he received notification a few days later that he didn’t qualify.

“They never told me why, just that they didn’t want me,” he said. He was disappointed because “it was a lot of money — enough to cover rent for a college student. And for something I already do.”