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. 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.

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. 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.

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.