The investigation into a listeria outbreak has been expanded to include three states where contaminated cantaloupe was consumed, Colorado health officials say.

Out of nine confirmed cases of the gastrointestinal infection in Colorado, two suspected cases in Texas and one in Nebraska, all of the patients recently ate cantaloupe.

Two people have died after being infected by the bacteria in Colorado, but only one of the deaths has been linked to a multi-state investigation focused on the cantaloupe.

Authorities don't know the source of the cantaloupe involved. People at high risk for listeria infections include pregnant women and those over 60. Deli meats and cheeses made with unpasteurized milk are more commonly linked to the infection, but outbreaks involving produce have occurred in the past.

Symptoms of the infection include fever and muscle aches, diarrhea, headaches, stiff neck and confusion, the Colorado health department said. The infection can also cause stillbirths and miscarriages.

Listeria outbreaks typically occur in the U.S. twice to four times a year. The investigation into the outbreak is still in the early stages and officials who identified the listeria strain in Colorado linked it to other states.

Since the outbreak has gone multi-state, the U.S. Centers for Disease Control and Prevention is coordinating the investigation in conjunction with the Food and Drug Administration and the Department of Agriculture.

People in the high-risk groups for infection should avoid unpasteurized soft cheeses, refrigerated smoked seafood and deli meats unless they have been reheated to 165 degrees Fahrenheit.

Healthy people are usually unaffected by the bacteria.