German outbreak of Escherichia coli O104:H4 associated with sprouts.

Background: A large outbreak of the hemolytic-uremic syndrome caused by Shiga-toxin-producing Escherichia coli O104:H4 occurred in Germany in May 2011. The source of infection was undetermined.

Methods: We conducted a matched case-control study and a recipe-based restaurant cohort study, along with environmental, trace-back, and trace-forward investigations, to determine the source of infection.