The US Centers for Disease Control and Prevention (CDC) on Friday said that four people were killed and 28 sickened due to contaminated prepackaged caramel apples, triggering a listeriosis outbreak.

Asking the public to remain cautious as the death toll is likely to rise, the CDC officials issued a health advisory against buying or eating any caramel apples.

The agency in association with the US Food and Drug Administration (FDA) and state health departments has launched an investigation into the outbreak.

Caused by the bacterium listeria monocytogenes, Listeriosis mostly affects people with weak or compromised immune systems, including pregnant women and elderly.

Some of the common symptoms include fever, stomach problems and muscle aching. Listeriosis also put the affected people at great risk of developing bacterial meningitis and encephalitis.

According to Dr. Willia Schaffner of Vanderbilt University Medical School, the incubation period of the infection can be as long as 70 days.

Schaffner says this clearly indicates that the death toll as well as number of sickened people who may have eaten contaminated caramel apples and have not yet shown the symptoms is likely to rise.

“We can anticipate that more illnesses will occur over time,” he noted.

The CDC report suggested that the cases of listeriosis have been reported in 10 states nationwide. Minnesota reported the first case which four people getting sickened and death of two, all after consuming the contaminated caramel apples.

According to the CDC officials who are investigating the outbreak, they have interviewed 18 infected people and found 15 to have consumed caramel apples.

The investigators said that the caramel apples prepared at home appeared to be safe and the fatal outbreak is unlikely to be linked with involvement of fresh fruit.