A mystery began unfolding at a correctional facility in Wyoming in the hours after lunch on October 11th, 2015. Inmates and staff became suddenly ill. Their stomachs cramped. They had gas, bloating, diarrhea, and burping. Some vomited. At the time, there were 254 inmates and 75 staff at the mixed-sex correctional facility — and eventually, 79 people reported getting sick. So the Wyoming Department of Health decided to investigate.

The Wyoming Department of Health decided to investigate

Today, the Centers for Disease Control and Prevention published the results of the investigation in its Morbidity and Mortality Weekly Report. The culprit? Rancid tortilla chips served at lunch. The Wyoming Department of Health interviewed residents and collected samples of their stool. They also tested the leftover food — which the correctional facility froze for "an extended period of time" — for chemical contamination and any possible added substances.

They found no parasites or other pathogens in the stool, and nothing seemed to be adulterating the food. But the investigators did find high levels of a compound called peroxide and a chemical called hexanal, which is in food additives, dyes, insecticides, and old fats that have gone bad. Together, the two molecules were a sign that the fat in the tortilla chips was rancid. It’s unusual for old, rancid fat to make people sick — the authors cite only one other example where 80 people were sickened by eating rancid biscuits at a wedding.

But it squares with the residents’ description of the chips. "Although the tortilla chips reportedly smelled and tasted foul, many persons consumed them," the authors report. That’s not surprising: the prison system across the US has recently come under fire for its practices of cutting costs and corners when it comes to food — with many residents reporting that they’re hungry.