Because food-related illnesses are so common, measuring whether food is getting safer or more dangerous is critical to public health. But it is also a daunting challenge.

The disease control agency uses three very different methods to routinely track sickness caused by food. Of these, FoodNet  the one whose most recent data were made public Thursday  is the most reliable, because government epidemiologists routinely survey more than 650 clinical laboratories that serve about 46 million people in 10 states. Such active surveillance tends to be more accurate than other tools that rely on voluntary, and generally spotty, reporting by doctors and hospitals.

Even so, FoodNet captures only a tiny slice of all those sickened by food. For a case to be included in FoodNet, someone must become sick enough to see a doctor, the doctor must be concerned or well trained enough to ask for and get a stool sample, and the laboratory to which the doctor sends the sample must be part of the government’s system.

Since 1996, when the system began, the burden of illness from campylobacter, listeria, shigella, E. coli O157 and Yersinia has decreased, although all of that decrease occurred before 2004. There has been no statistically significant change in the incidence of salmonella and cryptosporidium since 1996, and there has been a marked increase in cases of vibrio, a relatively rare disease mostly associated with raw oysters.

There are unexplained variations in infection rates among the 10 states in the FoodNet system. The incidence of campylobacter, for instance, is highest in California, while salmonella infections are highest in New Mexico and Georgia. Geographic differences in diet may be the cause, officials said.

Dr. Tim Jones, the state epidemiologist in Tennessee, said many of the easy improvements in the nation’s food-safety system had already been made.

“You can only tell people so much to wash their cutting boards and wash their hands,” Dr. Jones said. “I think we’re running out of things to do to make dramatic improvements.”

Even as the government issued its report, the Texas Department of State Health Services announced Thursday that it had assessed a $14.6 million fine against a Texas plant owned by the Peanut Corporation of America, the company involved in a national salmonella outbreak that sickened nearly 700 people. The fine resulted from violations that included unsanitary conditions and product contamination. The plant was closed in February.