This article presents a unique, large-scale and spatially explicit microsimulation model that uses official anonymized register data collected from all individuals living in Sweden. Individuals are connected to households and workplaces and represent crucial links in the Swedish social contact network. This enables significant policy experiments in the domain of epidemic outbreaks. Development of the model started in 2004 at the Swedish Institute for Infectious Disease Control (SMI) in Solna, Sweden with the goal of creating a tool for testing the effects of intervention policies. These interventions include mass vaccination, targeted vaccination, isolation and social distancing. The model was initially designed for simulating smallpox outbreaks. In 2006, it was modified to support simulations of pandemic influenza. All nine millions members of the Swedish population are represented in the model. This article is a technical description of the simulation model; the input data, the simulation engine and the basic object types.