Apocryphal tales of its meals causing the runs have plagued Taco Bell since time immemorial. But this latest parasitic outbreak is something else entirely.

As the Washington Post reports, federal and state authorities are investigating whether the fast food chain is behind the outbreak of explosive diarrhea cases across the country — further cementing its scatological legacy.

The illness, caused by the cyclosporiasis parasite, has sickened as many as 5,100 people across the country, according to the Centers for Disease Control and Prevention. In Michigan, which appears to be the epicenter of the outbreak, over 3,700 people have been infected, state authorities said.

Taco Bell’s suspected role in the outbreak comes after reports that the chain’s Michigan locations began pulling some of its ingredients. Signs in front of the restaurants stated that they were no longer serving lettuce, cilantro, onion, pico de gallo, and guacamole due to a “nationwide recall,” though what recall they were referring to is unclear.

Health officials have so far struggled to narrow down how the parasite is spreading. While usually found on fresh produce, symptoms can take weeks to manifest, making it difficult to trace back to the source.

Lettuce has emerged as one of the likeliest culprits. In an update this week, Michigan’s chief medical executive Natasha Bagdasarian said that “early information has shown lettuce as a common product that regularly comes up during the investigation.”

“Although we do not have a definite product identified as the source of the outbreak, we want to let Michiganders know what we have learned so far so they can take steps to protect their families,” Bagdasarian added.

Federal authorities haven’t singled out any veggies yet, though acting Food and Drug Administration director Don Prater acknowledged Michigan’s fixation on lettuce.

“What I can say at this point is that we’re continuing our traceback investigation on multiple produce items, including the lettuce,” he said, per WaPo.

So far, no nationwide recalls have been made related to the outbreak. One person familiar with the federal investigation, however, praised Taco Bell’s response. 

“It’s good the chain has already voluntarily stopped serving a number of items,” they told the newspaper. They added that some who became sick said they didn’t eat at the chain, suggesting that it’s not the only one responsible.

More on parasites: If You’ve Been Having Explosive Diarrhea, You May Want to Read This