In the big barn at the back of the property, Levy and 10 medical students, along with the Downings and their kids, built six wire pens, each equipped with gas heaters and independent food trays and water systems. Four were inside the barn, 50 feet apart; two more stood outside its thick timber walls. Then Levy went in search of chicks. To ensure there would be no contamination, nothing in the chickens’ systems that would slant the study results, he bought them from a company that supplied “pathogen-free” eggs to laboratories.

In July 1974, the day-old Leghorn chicks arrived in Sherborn and were stashed in one of the pens with a heat lamp and water and antibiotic-free food. When they were two months old, the experiment began. Levy divided the chicks into six batches, 50 per pen. At the local feed store, he bought two types of feed, one antibiotic free and another that was sold premixed with antibiotics. It contained oxytetracycline in a ratio of 100 grams per ton. Half of the birds, in three of the six pens, got the drug-free feed. The other half got the tetracycline-laced mix. Levy had several questions to answer. First, did the antibiotics in feed cause resistant bacteria to emerge or multiply in the chickens receiving it? Second, did that resistance cross from those chickens to the rest of the flock? And third and most crucial, could it make the leap from chickens to humans? For the experiment to establish everything that Levy planned, he needed to recruit people beyond the Downings to participate. At their invitation, he drove out to meet the neighbors. Joan and Richard threw a barbecue and invited the five families—10 parents, 14 kids—who lived up and down the road. After the hamburgers and hot dogs and corn were handed around, the Downing boys rolled a washtub over to make a podium. Levy had discussed with the parents what he planned to say, and they had reassured him that it was best to be straightforward. Still, he felt a little nervous as he climbed up on the tub.

“We’re asking you all to be part of an experiment,” he told the guests. The parents made murmurs of interest and shushed their kids. He described the puzzle of antibiotic resistance, how the chickens might help solve it, and that the Downings had agreed to assist. Then he got to the hard part.

“We’d like you to donate something that you have to science,” he said. The small crowd perked up with curiosity. He heard one woman say, “This is exciting!”

He took a deep breath. “Frankly,” he said, “we need your shit.” The silence was deadly. Then three-year-old Lisa scrambled up, round-eyed. “You want our poops?” she squeaked. That broke the ice. Everyone laughed, and they all agreed to help; none of the families backed out. And that in turn justified Mary’s substantial pay for managing the experiment. Her job was not just feeding and watering the chickens and swabbing their butts every seven days; it was also persuading and bugging and nagging her siblings and the neighbor kids, and her parents and the neighbor parents, to do their sampling duty. Every week, the Downings’ refrigerator shelf filled up with bags full of tubes, and then emptied again when Levy’s staff picked them up.

The results came quickly. Samples taken before the experiment began showed that very few bacteria in the guts of the chickens, family, and neighbors possessed genetic defenses against tetracycline. That was to be expected, given the random roulette of mutation. But once the drug-laced feed was administered, those bacteria multiplied in the birds harboring them and spread to birds that had started out clean. The first changes appeared in 36 hours, and within two weeks, 90 percent of the chickens were pooping out resistant bugs. The dose of antibiotics in the feed had killed the gut bacteria vulnerable to the drug but did not harm the ones that were protected by minor mutations—and those resistant survivors thrived and multiplied in the living space left empty when the other bacteria were killed. Researchers had assumed this was what occurred in animals given antibiotics, turning them into factories for producing resistant bacteria. But no one had measured it in the wild before, and no one had expected to see it happen so fast.

For a few weeks, the chickens getting drug-free feed remained clean of resistant bugs. Then things changed. First, the bacteria from the chickens getting the antibiotic-laced feed became resistant to multiple drugs—sulfas, chloramphenicol, streptomycin and neomycin, and two derivatives of penicillin—even though the feed contained only tetracycline. Then the multidrug-resistant bacteria appeared in the chickens that had never received that feed and had no contact with the birds that did. And soon after, the same multidrug-resistant bacteria showed up in the Downings’ poop as well.