The article looks at "EpiSims," an epidemiology simulation model created to study how social networks spread disease. Public health officials have to make choices that could mean life or death for thousands, even millions, of people, as well as massive economic and social disruption. That is why our group at Los Alamos National Laboratory set out to build EpiSims, the largest individual-based epidemiology simulation model ever created. Tracing the activities and contacts of individual disease victims remains an important tool for modern epidemiologists. After we began developing EpiSims in 2000, smallpox was among the first diseases we chose to model because government officials charged with bioterrorism planning and response were faced with several questions and sometimes conflicting recommendations. INSET: Overview/Simulating Society.