The data type most commonly used among surveillance systems is symptoms or diagnoses of patients from ED and/or physician office visits. Other types of data identified in that study include emergency call center and nurse advice lines. Other types of data being used include sales of over-thecounter health care products, prescriptions, telephone call volumes to health care providers and drug stores, and absenteeism. We have conducted studies demonstrating that the free-text chief complaint data that we use correlate with outbreaks. 21, 22 Design Objectives