The size and breadth of the Salmonella outbreak linked to Daniele Salami – 202 people in 42 states since last July – is a big part of the reason the source of the illnesses has been pinpointed.  The CDC has estimated that there are 75 million cases of food-poisoning in the U.S. each year.  Only a fraction of these illnesses are ever tied to a specific food item.

The epidemiological work of local and state health officials that respond to reports of illness is the first step to a connection.   State laws require that positive lab results for Salmonella  (among other pathogens) be reported to health officials.   Once alerted, health officials then contact the ill individual and attempt to determine his or her potential exposures.   In an isolated case, making determinations can be tricky.   When multiple people share the same bacteria strain however, looking for a common exposure factor can be more fruitful.   

A spokesman for the Washington State Department of Health explained some of its involvement in the national investigation in response to a report of a Jefferson County man who was one of 15 Washington residents reported as part of the outbreak:

Donn Moyer, state Department of Health spokesman, said the outbreak was unique in that a national study actually pinpointed the source of the salmonella montevideo contamination. "Often you don’t find the source," he said. "In this case you’ve got a number of people exposed to the same source. That’s why this was an outbreak."

Moyer stated that  a review of shopping receipts verified that 13 cases showed purchases of the same salami variety pack before getting sick.

The investigation also relied upon laboratory evidence, both from private and public labs, in Washington and elsewhere.  These tests not only confirmed the presence of Salmonella in the Daniele product, but also suggested the product was contaminated with multiple strains:

A private Washington state lab tested a different salami product, also produced by Daniele, and found it contained another type of salmonella.  A bacterial culture from the private lab, but not the salami product, was provided to the Washington State Department of Health for additional testing.Results from the state lab tests of that bacterial culture on Monday identified two types of the bacteria. One type matched the findings of the private lab, and the other matched the outbreak strain of salmonella montevideo.