MINNEAPOLIS (WCCO) — Health officials in two states have zeroed in on what’s making hundreds of people sick with a stomach bug. But investigators still don’t know what’s triggering cases in other states, including Minnesota and Wisconsin.

Pre-packaged salad is being blamed for the outbreak in Nebraska and Iowa.

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Officials are still trying to pinpoint specific brands.

So far, the bug called Cyclospora has made more than 350 people sick in 15 states.

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Health experts say washing your fruits and vegetables may be the best way to help protect yourself.

“The notion is that this type of contamination is on the outside of the product so washing them thorough reduces your risk of illness and it’s just good day in day out recommendation,” said Dr. William Schaffner, of Vanderbilt University Medical Center.

Washing hands, utensils, and surfaces with soap and water before and after you prepare a meal is also important.

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The FDA is trying to find out how the outbreak began and whether the cases are tied to one source, such as the same farmer or producer.