Of the nearly 400 people sickened across the country in a salmonella outbreak whose origins are still a mystery, about a dozen live in Pennsylvania, but only one hails from the western part of the state.

The single case reported to health authorities here was in Butler County. Other counties reporting cases are: Bucks, Chester, Clinton, Delaware, Lancaster, Lehigh, Luzerne, Montgomery and Philadelphia.

Investigators with the federal Centers for Disease Control and Prevention are studying cases in 42 states involving 388 people to determine what caused the current outbreak.

No one has died in Pennsylvania, where victims were between 3 and 73 years old. Most of the infections in the state were confirmed in mid- to late fall, with the latest confirmed Dec. 15, according to Stacy Kriedeman of the state Health Department.

Nationally, patients ranged from younger than 1 year old to 103 years old, and 18 percent of the 372 people who provided data were hospitalized. Illness began between Sept. 3 and Dec. 29 with most starting after Oct. 1.

The salmonella strain responsible for causing diarrhea, fever and stomach cramps across the nation -- it is called Typhimurium -- is the most common and is routinely reported to state health authorities.

As a result, state authorities did not realize the dozen cases in Pennsylvania were linked until they learned earlier this week of the national outbreak, Ms. Kriedeman said.

"Because it's so common it didn't pop up as unusual for us," Ms. Kriedeman said. "There was nothing that would have made us pause."

In 2007, the latest year for which figures are available, Pennsylvania recorded 1,948 confirmed and probable salmonella cases, up from 1,701 in 2006. Figures for 2008 were not available.

Ms. Kriedeman said the true number of salmonella cases each year in Pennsylvania could be dramatically higher -- possibly 20 additional cases for each one confirmed -- because of underreporting, incorrect test results or misdiagnosis resulting in no testing.

The Allegheny County Health Department reported 118 salmonella cases last year, up from 90 in 2007 but down from 123 the year before.

Investigators who interview people who had salmonella are trying to find foods they ate in common.

"CDC and its public health partners are vigorously working to identify the specific contaminated product, probably a food or foods, that is causing this outbreak," the CDC said.

"Outbreaks from a widely distributed contaminated product may cause illnesses across the United States, and the identity of the contaminated product is often not readily apparent."