Contaminated Roma tomatoes were the likely cause of a string of salmonella outbreaks that made 561 people sick in the United States and Canada last summer, U.S. health officials said on Thursday.

Salmonella is a common bacteria sometimes found in eggs, raw milk and raw meat. It typically causes diarrhea and other flu-like symptoms, and is rarely fatal.

Tomatoes have been linked to the bacteria in the United States since 1990 and are believed to have played a key role in three outbreaks that surfaced in North America last summer, said the Centers for Disease Control and Prevention in a report.

More than 150 people in 18 states, stretching from Kansas to New Hampshire, as well as the Canadian province of Ontario were hospitalized to be treated for salmonella infection, the Atlanta-based federal agency said.

Most had eaten at a U.S. delicatessen chain, according to the CDC, which did not identify the chain. Pre-sliced Roma tomatoes with the bacteria were found at one of the chain’s locations.

U.S. and Canadian health officials have been unable to determine the exact source of the suspect Roma tomatoes, which are smaller than the regular variety and oval in shape.

“Although a single tomato-packing house in Florida was common to all three outbreaks, other growers or packers also might have supplied contaminated Roma tomatoes that resulted in some of the illnesses,” the CDC said.

The outbreaks are a concern on both sides of the border because of the popularity of tomatoes in the food chain and a noticeable rise in the number of salmonella outbreaks linked to tomatoes in recent years.

A total of 1,616 such cases were reported to the CDC between 1990 and 2004, but officials say that the vast majority are not reported. When identified, salmonella infection is treated with antibiotics.

Tomatoes are usually grown in areas that are natural habitats for birds, reptiles and other species that carry salmonella. The bacteria are believed to enter tomato plants through roots or flowers and the fruit itself through small cracks in the skin, stem scar or plant.

Eradicating salmonella once it gets inside a tomato is almost impossible without cooking.