The Centers for Disease Control warned U.S. consumers on Friday not to eat pre-packaged, commercially produced caramel apples after health officials linked the treats to a recent listeria outbreak that has killed five people and resulted in the hospitalization of 21 others. As of Thursday, 28 people have been infected by listeria in an outbreak across 10 states, according to the CDC’s notice.

The CDC is collaborating with public health officials from several states along with the Food and Drug Administration to investigate the illnesses, the agency said. Investigators are working to identify the source of contamination, as well as to determine the specific brands or types of prepackaged apples linked to the outbreak.

Listeria infection (listeriosis) is a food-borne bacterial illness that primarily affects people with weakened immune systems, according to the Mayo Clinic. It is most commonly contracted by consuming improperly processed deli meats and unpasteurized dairy products and could have life-threatening complications for pregnant women and people with impaired immune systems. Symptoms of the infection include fever, muscle aches and nausea and can lead to convulsions, loss of balance and a stiff neck once it spreads to the nervous system.

Of the 28 reported infections reported by the CDC Friday, nine were pregnancy-related, meaning that they occurred in a pregnant woman or a newborn infant, the agency said. Of the 18 people interviewed by the CDC, 15 reported eating commercially produced, pre-packaged caramel apples.

The Minnesota Department of Health said that four of the cases, including two of the fatalities, occurred in the state in late October and November, according to Reuters. Those cases were linked to supermarket chains including Supervalu Inc.'s Cub Foods, Kwik Trip and Mike's Discount Foods, where infected patients reported buying caramel apples. Though the stores have reportedly pulled the products, health officials are still concerned that people who purchased them earlier may still have them at home.