In 2004, the Marion County Public Health Department (MCPHD), which serves a county population over 890,000, began using a real-time syndromic surveillance system, ES-SENCE (Electronic Surveillance System for the Early Notification of Community-based Epidemics) to assist in detecting possible disease outbreaks. Today, about 1600 emergency department visits occur daily in Marion County's 14 emergency departments. Epidemiologists from MCPHD have contributed to the city's extreme temperature plans for the last few years. While most of the previous increases in heat-related illnesses in Marion County have been attributed to prolonged heat exposure in connection with local auto races, the county had not activated the county wide emergency response plan in several years. From Tuesday, July 19 through Friday, July 22, 2011, the Marion County Extreme Temperature Plan was put into action in response to several days of a high heat index.