While public health authorities haven’t conclusively proven which foodborne pathogen made hundreds of people sick after Thanksgiving dinner at a restaurant in western New York, they do know which food item made people sick. Gravy was served at a low temperature that allowed Clostridium perfringens bacteria to thrive.

What’s C. perfringens? It’s a pretty common foodborne illness, with an estimated 1 million cases a year in the United States. The symptoms consist of diarrhea and abdominal cramping that begins abruptly between six and 24 hours after consuming contaminated food. The illness usually resolves in less than 24 hours, though infection can drag on for up to two weeks and cause dehydration.

While you’ve probably had C. perfringens without knowing its name, it can be deadly: food contaminated with the bacteria caused the deaths of three elderly guests at a church-sponsored Thanksgiving dinner in California this year.

While the authorities haven’t conclusively confirmed that C. perfringens was the cause of this outbreak that made hundreds of people sick, they have been able to link the illness to gravy served during the Thanksgiving meal.

The problem, according to the Monroe County Department of Health’s spokesman, was that the restaurant simply served more people than it should have during the holiday, and one of the things that it served was gravy that may not have been heated all the way through.

“It’s pretty clear that the result of the sheer volume of food they served that day was that it was probably very difficult, if not impossible, to keep everything at [a safe] temperature,” the health department spokesman told the Rochester Democrat and Chronicle. “We stated to them that trying to serve that many people is just imprudent, to say the least.”

The restaurant’s owner agrees that underheated gravy was the likely cause. The health department has approved the restaurant to re-open on Tuesday, Dec. 27. It’s been thoroughly cleaned, and leftover gravy will definitely not be on the menu.

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