The Prevalence of Shigella in Food and Elsewhere
Most Shigella infections occur sporadically, but large Shigella outbreaks have been traced to contaminated food and water.
The CDC estimates that 450,000 total cases of shigellosis occur in the U.S. every year. [4, 11, 21] Shigellosis is also characterized by seasonality, with the largest percentage of reported cases occurring between July and October, and the smallest proportion occurring in January, February, and March.  Sporadic (or non-outbreak) infections account for the majority of cases and, in general, the exact means by which persons are infected (risk factors) are not yet well documented or understood. [21, 36]
Shigella is an especially common cause of disease among young children, in large part because it is difficult to control the spread of the bacteria in daycare settings. [16, 28] The symptoms of shigellosis vary so widely that children shedding Shigella in their stool may exhibit no symptoms of infection. A person infected with Shigella can be asymptomatic (show no symptoms of illness), suffer from moderate to severe diarrhea, or suffer complications up to and including death. [11, 17, 26]