Federal health officials have fine tuned how they measure deaths and illnesses caused by food poisoning, resulting in fewer estimated incidents but a clearer picture of a group of diseases that account for billions of dollars of health-care costs.

Nearly one in six Americans is stricken by food poisoning each year and more than 3,000 die, say two studies by the U.S. Centers for Disease Control and Prevention released on Wednesday.

More than three-quarters of the illnesses are never traced to a particular pathogen. Most that are linked to a pathogen are caused by norovirus, which is responsible for an estimated 5.5 million infections a year. Second is Salmonella, which has been behind several outbreaks in recent years, cropping up in unexpected sources such as peanuts.

Salmonella, a bacterial infection causing fever, diarrhea, and abdominal cramps, lands more people in the hospital and kills more than any other identified foodborne illness, the CDC said.

The new estimates were released as Congress is weighing a food-safety bill that would allow the Food and Drug Administration to step up inspections and order food recalls. Advocates hope the bill will pass this week before Congress takes its recess.

The newly released numbers are lower than prior estimates that foodborne illnesses sicken 76 million people a year and kill 5,000 in the U.S. Those were based on a 1999 report.

But the change doesn't mean the scope of food poisoning has declined, said CDC official Chris Braden. Rather, he said, they are due to changes in methodology and data analysis.

Data from a CDC food-surveillance system show a 20% reduction in illnesses over the past decade caused by the pathogens it tracks. But that system follows a smaller number of pathogens than were included in these estimates, Dr. Braden said.

"A lot more work still needs to be done," he said. "If we could reduce foodborne illnesses by just 1%, we could keep about half a million people each year from getting sick from foods that they eat."

Write to Betsy McKay at betsy.mckay@wsj.com