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It’s the middle of the day for Eric, a 24-year-old research assistant at the Massachusetts Institute of Technology, and nature is calling.

Eric leaves his job and hops a train. Then a bus. Then he walks some more. He passes countless toilets, and he needs to use them, but he doesn’t.

Eventually, Eric arrives at a nondescript men’s room 30 minutes away from MIT. A partition separates two toilets. There’s a square-tiled floor like in any public restroom. It’s unremarkable in every way, with one exception: A pit stop here can save lives.

Eric hangs a plastic collection bucket down inside the toilet bowl and does his business. When he’s finished, he puts a lid on the container, bags it up and walks his stool a few doors down the hall to OpenBiome, a small laboratory northwest of Boston that has developed a way to turn poop from extremely healthy people into medicine for really sick patients.

A lab technician weighs Eric’s “sample.” Over the past 2½ months, Eric has generated 10.6 pounds of poop over 29 visits, enough feces to produce 133 treatments for patients suffering from Clostridium difficile, an infection that kills 15,000 Americans a year and sickens half a million.

To donate, Eric had to pass a 109-point clinical assessment. There is a laundry list of factors that would disqualify a donor: obesity, illicit drug use, antibiotic use, travel to regions with high risk of contracting diseases, even recent tattoos. His stools and blood also had to clear a battery of laboratory screenings to make sure he didn’t have any infections.

After all that screening, only 3% of prospective donors are healthy enough to give. “I had no idea,” he says about his poop. “It turns out that it’s fairly close to perfect.”

And that, unlike most people’s poop, makes Eric’s worth money. OpenBiome pays its 22 active donors $40 per sample. They’re encouraged to donate often, every day if they can. Eric has earned about $1,000.

“It takes us a lot of time and effort to find these donors,” says OpenBiome’s research director, Mark Smith. “When we do find them, we want to keep them as engaged as possible and really want to compensate them for their time.”

Why is Eric’s poop so valuable?

A hundred trillion bacteria live inside your gut, some good, some bad. When patients take antibiotics for infections, sometimes they fail to work; good bacteria gets killed off while bad bacteria — C. difficile — grows unchecked.

The life-saving bacteria from the guts of people like Eric can help. When their healthy microbes are placed inside the intestines of a sick person they can chase out harmful C. difficile bacteria. It’s called a fecal transplant. The treatments are administered bottom-up, through a colonoscopy, or top-down, through a tube in the nose.

OpenBiome’s poop donors have created about 5,000 treatments, and the organization says the results have been stunning. Stinky human waste is an astonishingly simple cure: 90% of the patients get better.

“They’ll actually have this really transformational experience where they’ll be going to the bathroom 20 times a day and then have normal bowel movements sort of immediately or the next day,” Smith says.

The organization’s fecal transplants cost $385 to purchase and are providing a treatment to more than 350 hospitals in 47 states.

At OpenBiome’s lab, technician Christina Kim, working under a fume hood that sucks up odors, pulls the lid off Eric’s collection bucket and demonstrates how she turns poop into the life-saving treatment.

“It’s nice that this room is actually closed off because this is where the smelly part happens,” she says.

She examines the consistency of today’s offering. A nearby chart has descriptions and illustrations for seven types of stools. It was developed by a hospital in Bristol, England, as a visual guide.

Not all poop is acceptable.

Types one or two, defined by the Bristol Stool Chart as “like nuts” or “lumpy,” are too dry to process into a treatment.

If a donor’s stool is “mushy” or “watery” — that’s a type six or seven — then it can’t be used because it could be a sign the donor has a gastrointestinal infection.

The perfect poop is type three, which is “like a sausage but with cracks on its surface;” type four, which is “like a sausage or snake, smooth and soft;” or type five, “soft blobs with clear-cut edges (passed easily).”

“It’s actually an established medical chart,” Kim says with a chuckle. “It’s very important.”

Maybe it was the hot sauce he used on his quinoa and cheddar cheese casserole last night, or the banana and peanut butter he ate with a bowl of bran flakes and almond milk for breakfast, but Eric’s stool is type five, just barely acceptable for processing.

Kim scoops the feces into a clear plastic bag and adds a saline solution. For two minutes the bag sloshes around inside a machine called the “jumbo mix.” The fiber in Eric’s stool is filtered out, and what’s left behind is a liquid teaming with helpful bacteria.

With a pipette, Kim transfers the watery remnants of Eric’s poop into 250 ml plastic bottles. On average, one stool donation fills four, but today Eric’s impressive half-pound sample fills seven. One bottle equals one treatment.

The 133 treatments Eric has provided won’t be distributed until he’s passed a secondary healthy screening. For now, they sit frozen in quarantine inside a giant freezer.

Most donors head on their way after handing over their sample, but during today’s visit Eric asks if he can see the treatments he helped create.

Cool air blasts his face as Kim opens the freezer. His jaw drops at the sight of his icy brown bottles, which look like frozen chocolate milkshakes. The bacteria inside them is still alive, cryogenically preserved at -112°F.

“That’s fantastic! Holy cow!” Eric says, beaming. “It’s unreal. I never thought I would be staring at my poop frozen in a freezer destined to help people across the country. It’s really cool.”

But did he do it for the money? The ridiculously easy money?

“Not at all,” he says. “It’s a nice perk, of course.”

If you’re inspired to donate like Eric, you have to live in the Boston area. And you may have to wait. Some 6,000 people have already signed up. OpenBiome usually invites about 50 people for interviews every week.

“It’s easier to get into MIT and Harvard than it is to get enrolled as one of our donors,” Smith says. “A lot of our donors are pretty excited to take something they do every day otherwise and save people’s lives with it.”