Caramel apples are a timeless fall treat, but it’s a good idea to make sure they are not left out on the counter overnight.

According to a report from the Washington Post, a 2014 Listeria monocytogenes outbreak killed seven people, caused one miscarriage and infected a total of 35 people. Investigators connected the outbreak to a specific brand of packaged caramel apples.

Researchers at the University of Wisconsin-Madison’s Food Research Institute were puzzled by the 2014 outbreak, as neither heated caramel nor fresh apples were suitable environments for Listeria bacteria to spread. They hypothesized that something about the delicious recipe could be creating the right habitat for the deadly germs.

The researchers realized that the bacteria must have been getting into the apples some other way. The culprit, they found, was the wooden sticks inserted into the tops of the apples for the purpose of dipping them into the molten caramel.

When the stick punctures the skin of the apple, it causes a small amount of juice to leak through the hole. This creates a microscopic pocket between the apple and the caramel where Listeria can multiply.

The risk was increased further when caramel apples were left unrefrigerated. At room temperature, the bacteria grew at a much higher rate.

The apples in the study were intentionally coated with Listeria samples before testing began. Homemade caramel apples are probably safe to make and consume, as long as the apples are thoroughly washed and the kitchen is thoroughly cleaned.

With store-bought caramel apples, however, it’s more difficult to know what you’re getting. The 2014 outbreak was traced to caramel apples produced by just one company, the Bidart Bros. Investigators across several states linked the deadly outbreak to these commercially produced caramel apples.

The outbreak affected people in Arizona, California, Colorado, Minnesota, Missouri, Nevada, New Mexico, North Carolina, Texas, Utah, Washington, and Wisconsin from October 17, 2014 to January 6, 2015. There were eleven illnesses linked to pregnancies. Among the illnesses that weren’t related to pregnancies, the median age of patients was 62 and 33 percent were female. The youngest victim was 7 years old.

28 of the ill people told investigators that they had eaten a prepackaged caramel apple before they became sick. After the Bidart Bros. were established as the primary source of the Listeria, the company voluntarily recalled granny smith and gala apples after a test of their processing facility came out positive for listeria infection. Subsequent tests revealed that the strain causing the illnesses were similar to strains present in their facilities, and after a proper decontamination their apples were deemed safe by the FDA once again.