This will turn your stomach.

As an outbreak linked to McDonald's Quarter Pounder sickens dozens of people with E. coli, food safety experts reveal which items to avoid to avoid foodborne illnesses at the supermarket.

At the top of the list are packaged, pre-washed vegetables.

Barbara Kowalcyk, director of the Institute for Food Safety and Nutritional Security at George Washington University, told NBC News that products that claim to be pre-washed – such as lettuce, vegetables or even cut fruit – should be avoided because they may be grown on the ground for bacteria.

Instead, she recommends buying whole heads of lettuce, washing them yourself with water and patting them dry with paper towels.

“Any leaf that's torn or damaged, I just throw it away because that's where the bacteria can get in,” she explained, adding that she also avoids sprouts and melons. “And I usually cut off the outer leaves and throw them away.”

The melon's webbed rind can trap bacteria, added Darin Detwiler, a food regulatory policy professor at Northeastern University.

“It's one of the deadliest foods of all things,” he told NBC News. “Cantaloupe has a perfect pH and you can’t clean the outside enough.”

Don Schaffner, a professor of food science at Rutgers University, warned that most fruits — especially those that have been stored for long periods of time — also pose a risk.

“I do a lot of workshops and often there's sliced ​​fruit on the breakfast buffet, and that fruit just sits there all day,” he told NBC News. “We have conducted research to show that the external appearance does not change, but bacteria can grow that are no one’s business.”

According to the Centers for Disease Control and Prevention, symptoms of food poisoning from bacteria such as listeria or E. coli — which cause infections in the gastrointestinal tract — include nausea, vomiting, fever, abdominal pain and diarrhea. Some cases can even result in hospitalization or be fatal.

The E. coli outbreak at McDonald's has already killed one person in Colorado and spread to several states, as the fast-food chain's supplier recalled yellow onions “out of an abundance of caution,” prompting other restaurants to do the same .

Schaffner explained that the more food that is brought from the farm to the table, the greater the risk of bacteria entering it that can then make you sick.

“Certainly the more you manipulate, the more opportunities there are for something to go wrong,” he said.