The Illinois Salmonella outbreak at Subway restaurants is certainly the beneficiary of multiple epidemiological factors that have helped the Illinois health department identify the outbreak, and hopefully stop the spread of illness.

1.  The strain of Salmonella involved in the Subway Salmonella outbreak is called Salmonella Hvittingfoss.  This is a rare strain of Salmonella; in fact, it is one of approximately 2,000 different strains of Salmonella.  Hvittingfoss is seen only a couple of times annually in the State of Illinois, according to health officials.  Thus, when a bunch of people began testing positive for the rare strain of Salmonella at roughly the same time and in many different counties throughout the central part of the state, health officials knew that an outbreak was underway. 

2.  Another circumstance that was surely helpful to investigating health officials in the Subway salmonella outbreak is that, for now at least, the outbreak appears to have been limited to only Subway restaurants.  Had people been exposed to the rare strain of bacteria at multiple different restaurants, it would have been much more difficult to contain the spread of disease, as there would have been a less defined pattern of illness. 

3.  Confirmed Salmonella illnesses in this outbreak occurred between May 14 and 25, when health officials nationally, and particularly in the upper midwest, were undoubtedly on alert for Salmonella and other foodpoisoning outbreaks linked to produce.  Other states in Illinois’ neighborhood, including Michigan and Ohio, had recently been embroiled in a large E. coli outbreak linked to romaine lettuce; in early May, Fresh Express lettuce products caused an undisclosed number of illnesses in the "upper midwest"; and on May 24, Fresh Express announced a recall of romaine-based lettuce and salad products due to potential Salmonella contamination.