Foodborne illness investigators know to expect a bacteria like Listeria monocytogenes on just about any food product. But even so, caramel apples were not on anyone’s radar when it became clear they were linked to a Listeria outbreak that has been associated with five deaths and at least 28 illnesses in 10 states. The U.S. Centers for Disease Control and Prevention (CDC) first learned of a cluster of related Listeria illnesses in November, but it took until this week for a multi-state public health investigation to determine that the likely source of the infections was caramel apples distributed under at least two brand names. So far, Food Safety News has learned few details about the patients who have died in the outbreak; however, CDC Epidemiologist Brendan Jackson confirmed that none were children. Nine of the 28 reported illnesses occurred in pregnant women, although none of those resulted in the loss of the child. While four of the five deaths were directly caused by Listeria, the fifth occurred in someone who was immunocompromised and already suffering from other life-threatening conditions. Part of the challenge with this outbreak investigation has been the relatively long incubation time for Listeria to cause symptoms of illness, said an epidemiologist with the Minnesota Department of Health. Though the first illnesses began in mid-October, health investigators have only recently been aware of the full extent of the outbreak. The discovery of caramel apples as the likely cause came even later, the epidemiologist added. One patient happened to mention eating a caramel apple, and so an investigator asked another patient, who also happened to have eaten one. To date, 15 of 18 patients interviewed have confirmed they ate prepackaged caramel apples prior to falling ill — a very statistically significant proportion given the relatively small subset of caramel-apple consumers within the general population. Investigators are still working to determine exactly how the caramel apples might have become contaminated, considering that the outbreak has not been associated with any non-caramel apples. As of Friday night, no recalls have been announced, and authorities are not ready to name all of the brand names involved. Based on information from the Minnesota Department of Health, the only known related brands are Carnival and Kitchen Cravings. According to the Washington State Department of Health, implicated caramel apples were sold in supermarkets in single or 3-pack plastic clamshell packages. CDC is recommending that the public not eat any commercially produced, prepackaged caramel apples at this time. Food Safety News reporter Lydia Zuraw contributed to this report.