WEST POINT, Neb. (AP) — Authorities are investigating an outbreak of salmonella in northeast Nebraska.

Officials say 15 cases in West Point have been confirmed. Six more are suspected.

The source of the outbreak hasn’t been confirmed. Medical experts say salmonella is caused by bacteria that live in the intestinal tracts of humans and animals. It is usually spread by eating contaminated food, including beef, poultry, milk, eggs and vegetables. Thorough cooking kills salmonella.

Symptoms include fever, diarrhea and stomach cramps.