Distribution of holding-level and individual-level prevalences. Stochastic disease spread modelling of verotoxigenic Escherichia coli O157:H7 in the complete Swedish cattle population. Calves are younger than 120 days, young stock are between 120 and 365 days, and adults are older than 365 days. The holding prevalence was calculated among the number of active holdings i.e. holdings having at least one animal. The prevalences were determined from 1000 trajectories.