ATLANTA -- Health officials are investigating a salmonella outbreak that has reportedly sickened nearly 400 people in 42 states, but they don't know how the bacteria are spreading.

The U.S. Centers for Disease Control and Prevention has not named all the states.

Ohio officials say at least 50 people there have been sickened by salmonella since October. California officials report 51 cases as of last week. Michigan had 20 cases and seven people there were hospitalized.

Most people infected with salmonella develop diarrhea, fever and abdominal cramps 12 to 72 hours later. The illness usually lasts four to seven days. Most people recover without treatment.

Officials say steps to protect against illness include careful handling of raw meat and frequent hand washing.