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As we track various E. coli and salmonella cases over here at The Takeout, it’s often fascinating how the CDC and other agencies can function as institutional Columbos, tracing these outbreaks to their points of origin. That recent McDonald’s salad recall just got tracked to California, for example. So it’s a bit unusual/alarming when the source for a considerable outbreak has yet to be discovered. But the recent incident at an Ohio’s Chipotle that sickened hundreds of people still has no known cause.

According to Columbus Business First, “Health officials still have no explanation for what may have sickened hundreds of customers at a Chipotle Mexican Grill in Central Ohio last month.” The Delaware General Health District notes that “tests by the Ohio Department of Health of leftover food from the Powell restaurant found no evidence of salmonella, shigella, E. coli or other forms of bacteria,” the usual culprits in such an outbreak.

Warning: There are also stool samples in this story. And even tests of those particular samples from the ill Chipotle customers failed to reveal evidence of bacteria or norovirus. With 644 people reported ill from eating at that Ohio Chipotle during a specific week, even with that large sample size, no definitive results have been reported. The Delaware General Health District states that some results are still pending.

One branding expert suggested that this latest outbreak won’t have much of an effect on the Chipotle brand because “In most situations this would be a horrifying kiss of death, but with Chipotle it is baked into the brand’s story” due to previous illness incidents. We’ll see how that all plays out; in the meantime, Forbes reports that Chipotle is now testing bacon, nachos, and late-night deals in certain cities.