Have you ever heard of “cannibal sandwiches?” Well, for mid-west residents, it’s a holiday menu staple. The sandwiches, made with raw beef, have been linked to cases of foodborne illnesses since the 1970s.  The Centers for Disease Control and Prevention (CDC) has issued a stern warning to those wishing to continue taking part in the tradition: please stop.

"Despite ongoing outreach efforts addressing the dangers associated with consuming undercooked or raw ground beef, this regional holiday tradition continues to be associated with outbreaks," the CDC said, according to Reuters.

According to The Associated Press, cannibal sandwiches are composed of raw, lean ground beef. The meat is seasoned with salt and pepper and served on a rye cocktail bread with a slice of raw onion. Sometimes the sandwich is prepared with raw egg also mixed in with the meat. Milwaukee historian John Gurda told the AP that he served the sandwich at his wedding in 1977. Gurda says the tradition of serving the sandwiches was created in German and Polish communities in Milwaukee and has continued through today. "It's like a coarse pate and when you put the onions on, there's a crunch as well and that kind of cuts the softness," said Gurda, according to the AP.

But what Gurda sees as a festive holiday treat may actually be causing illnesses across the Midwest. The CDC has linked at least 50 cases of foodborne illnesses in 1972, 1978, and 1994 to cannibal sandwiches.  In field notes released Friday, the CDC says that E.coli outbreaks have been associated with the “seasonal consumption of raw ground beef” between December 2012 and January 2013. E.coli bacteria live in the intestines of healthy people and animals. However, when people are exposed to E.coli in contaminated water or food, it can cause infection. Symptoms of E.coli infection include diarrhea, abdominal cramping, and nausea. Most people recover from an E.coli infection within a week, but in severe cases, patients may develop hemolytic-uremic syndrome, a deadly form of kidney failure.

"The big message is that people need to cook their food properly and make sure they're taking temperatures of their meat," said Carol Quest, a Wisconsin health department director, according to the AP.

The CDC hopes to educate ground beef retailers in the Midwest —  especially Wisconsin — of the potential severe illness associated with raw beef consumption. The agency will encourage retailers to tell their customers to cook the beef to avoid potential outbreaks.

“To prevent illness, ground beef should be cooked to an internal temperature of 160°F (71°C), as measured with a food thermometer, before consumption,” the CDC said.