On May 25, 2010, the Sangamon County Department of Public Health reported receiving six Salmonella reports between May 21 and May 25. On May 26, personnel from the Communicable Disease Control Section (CDCS), Division of Infectious Disease (DID), and Illinois Department of Public Health (IDPH) noted an increase in Salmonella cases in the daily report from the Illinois National Electronic Disease Surveillance System (INEDSS) in small counties in central Illinois. The IDPH Springfield laboratory then identified 44 Salmonella isolates reported during the week of May 17, as compared to 25 isolates in the week of May 10.

On Thursday, May 27, the CDCS was notified by the IDPH Springfield laboratory of 10 cases of Salmonella Hvittingfoss, an uncommon serotype of Salmonella. An investigation was then initiated to examine this increase in Salmonella Hvittingfoss.

Health department officials eventually identified 109 confirmed cases of Salmonella Hvttingfoss among Illinois residents, or among people traveling through Illinois. Isolates from 107 cases had identical PFGE patterns, and two had closely related patterns. Onsets of illness were from April 25 through June 20, 2010.

Further investigation revealed a strong connection between Salmonella Hvttingfoss and the Subway restaurant chain. Individuals determined to be confirmed cases reported eating at 49 individual Subway restaurants in Illinois between May 5 and June 4, and ninety-six percent of cases reported eating at a Subway location within the distribution area of one Subway product distributor. Additionally, of those infected, twelve individuals were food handlers of the Subway restaurant chain, and who worked at different locations.

Epidemiologic analyses found a strong statistical link between Salmonella Hvittingfoss cases and consumption of foods from Subway restaurants with an odds ratio of 19.2 (95% confidence limits: 3.9, 155; p-value = 0.00008).