Influenza is a major cause of mortality. In developed countries, mortality is at its highest during winter months, not only as a result of deaths from influenza and pneumonia but also as a result of deaths attributed to other diseases (e.g., cardiovascular disease). Understandably, much of the surveillance of influenza follows predefined geographic regions (e.g., census regions or state boundaries). However, the spread of influenza and its resulting mortality does not respect such boundaries.