ATLANTA (CNN) -- A toasted oat cereal appears to be the source of an outbreak of a rare type of salmonella poisoning that has made 188 people sick in 11 Midwestern and Northeastern states, according to the federal Centers for Disease Control and Prevention.

Forty people have been hospitalized, but no deaths have been reported, said the CDC, which first reported the outbreak last week.

The cereal, marketed under the Millville brand, is distributed by Aldi supermarkets, CDC spokesman Tom Skinner said. The CDC is warning people not to eat Millville toasted oat cereal until further investigations are complete.

The Illinois-based chain has 505 stores around the country. It took the cereal off the shelves on Monday after receiving a call Sunday from the Illinois Department of Public Health, indicating that the cereal seemed to be responsible for the outbreak, said Philip Whitfield, a spokesman for Aldi.

The supermarket is asking anyone who purchased the cereal to return it for a refund.

The strain of salmonella, called agona, is normally found in chicken, turkeys, cattle and pigs. Skinner said no one knows how it got into the cereal.

"That's the million-dollar question," he said.

Whitfield said the cereal is manufactured at a Malt-O-Meal plant in Northfield, Minnesota. Other cereal products made at the same plant have not been implicated, but the CDC is continuing to investigate them. There is no evidence that toasted oat cereal products from other manufacturers are linked to any illnesses, according to the CDC.

Illinois has been hardest hit, with 46 cases since April 1. Sixteen of those are in children under 10 years of age, said P.J. Burtle-McCredie, spokeswoman for the Illinois Department of Public Health.

Like other salmonella infections, agona causes diarrhea, fever and abdominal cramps 12 hours to 72 hours after infection. The illness usually lasts four to seven days, and most people recover without treatment.

However, the infections can be fatal among infants, the elderly and people with compromised immune systems.

Agona salmonella is fairly rare, accounting for only about 500 to 1,000 of the 2 million to 4 million annual salmonella cases in the United States, according to the CDC.

In addition to Illinois, the outbreak has affected people in Indiana, Michigan, Ohio, Missouri, New York, Pennsylvania, Iowa, Kansas, West Virginia and Wisconsin.