CAMBRIDGE, MASS. — Around noon on a recent Friday, Donor Five, a healthy 31-year-old, walked across M.I.T.’s frigid, wind-swept campus to a third-floor restroom to make a contribution to public health.

Less than two hours later, a technician blended the donor’s stool into preparations that looked like chocolate milk. The material was separated and stored in freezers at an M.I.T. microbiology lab, awaiting shipment to hospitals around the country. Each container was carefully labeled: Fecal Microbiota Preparation.

Nearly a year ago, Mark Smith, a 27-year-old doctoral candidate, and three colleagues launched OpenBiome, the nation’s first human stool bank. Its mission: to provide doctors with safe, inexpensive fecal material from screened donors to treat patients with Clostridium difficile, a gastrointestinal infection that kills at least 14,000 Americans a year.

“People are dying, and it’s crazy because we know what the solution is,” Mr. Smith said. “People are doing fecal transplants in their basements and may not be doing any of the right screening or sterile preparation. We need an intermediate solution until there are commercial products on the market.”