Listeria is ubiquitous in the environment and can live in food processing plants, even in cold temperatures, such as those in refrigerators. It is killed by cooking and pasteurization. It is also found in soil and water and in animals such as poultry and cattle, and can be present in raw milk or foods made from raw milk.

Minnesota health officials have determined that two people died and two others were sickened after eating contaminated prepackaged caramel apples, according to the Associated Press. The four were between 59 and 90 years old.

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The CDC said nine of the illnesses across the nation occurred in pregnant women or newborns. The disease particularly affects older adults, pregnant women, newborns and people with weakened immune systems. Three people have come down with meningitis.

The other states where infections have been found are: Arizona (4), California (1), Missouri (5), New Mexico (5), North Carolina (1), Texas (4), Utah (1), Washington (1) and Wisconsin (2).

Listeria outbreaks are rare but dangerous. In 2011, listeria in cantaloupes killed 33 people and sickened 147 in 28 states, according to the CDC. In 2012, 22 people were infected and four died in an outbreak attributed to a brand of ricotta cheese imported from Italy, the agency said.

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Brendan Jackson, a medical officer for the CDC, said that several states began noticing listeria cases early in November and that the agency launched an investigation of the outbreak, which was attributed to bacteria with two different genetic fingerprints. Caramel apples are not on the list of foods that investigators typically consider when searching for listeria, and the bacteria has a long incubation period, both of which prolonged the effort, Jackson said. Listeria can take as long as 70 days to incubate and sicken someone, especially a healthy person.

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"It really took quite a bit of time to identify caramel apples," he said. "It's all about finding that needle in the haystack. And you're asking people what they ate weeks and weeks before." Eventually, he said, two people in Texas mentioned eating caramel apples and others with listeria were subsequently asked about that snack.

Even now, Jackson said, investigators haven't traced the outbreak to its source. But they believe people still may have the caramel apples in their homes, and put out the notice in hopes of preventing any additional consumption of them, he said.

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Bill Marler, a food safety attorney in Seattle who is representing many of the victims of the 2011 listeria outbreak, said he believes it's likely that the contamination occurred at a food processing plant. Once listeria takes hold in such a plant, it can be difficult to eliminate, he said.

"Could it have been on the apples that came into a facility? Absolutely," he said. "But they were probably washed." The caramel itself, he said, seems unlikely to harbor the bacteria. "I certainly think the more likely scenario is the manufacturing facility where this was produced was contaminated," he said.