BioSense is a US national automated surveillance system that receives data from various sources and makes them available for public health use. The data may be viewed simultaneously by local, state, and federal public health offi cials through the Internet-based BioSense Application, which may be accessed on a jurisdiction-specifi c basis through the Centers for Disease Control and Prevention (CDC) Secure Data Network (3) . Data received include coded fi nal diagnoses and free-text chief complaints, which are assigned as appropriate to >1 of 11 syndrome groupings representing general illness categories such as respiratory and gastrointestinal illnesses (4) and to >1 of 78 subsyndromes representing more specifi c categories such as asthma or cough (5) . To identify days when disease indicator activity is higher than expected, BioSense uses a modifi ed version of the C2 algorithm, 1 of 3 algorithms (C1, C2, and C3) developed for the Early Aberration Reporting System (EARS) (6, 7) .