The source of infection for sporadic cases of listeriosis is rarely identified for several reasons: a large number of potential food exposures can occur during the lengthy incubation period of 3–70 days, patients are often elderly or frail and their food history recall may be poor, or the patient may have died and no food history is available. In addition, microbiological evidence is limited as food samples for sporadic cases are not often obtained. Poor understanding of the background prevalence of L. monocytogenes strains is also a factor. Investigations may also be hampered by the complexity of food-distribution networks