About this time of year, 4 years ago, people in Germany began falling ill with diarrhea, nausea and fever. At first it looked like a small outbreak, though a troubling one: One out of every three victims needed to be admitted to the hospital. Then the outbreak expanded, and expanded again. By June, more than 1,800 people had fallen ill with the bacterium E. coli  O104:H4, a strain that produces a destructive toxin. More than 500 people had been hospitalized, and 20 died. The vast scale of the outbreak spurred an international hunt for the culprit food, bringing together not only professional disease detectives from Europe and the United States but crowdsourced genomics contributions from as far away as China.

By July 2011, with