After the data cleansing, we categorized ED visits into 11 different syndromic groups important in Taiwan. There were: (1) fever, (2) respiratory, (3) skin, (4) neurological, (5) upper gastrointestinal (GI), (6) lower GI, (7) haemorrhagic, (8) influenza-like illness (ILI), (9) asthma, (10) enterovirus-related infection (EVI) syndrome, and (11) syndrome for severe illness or death. Since only about 25% of all chief complaints were written fully in English and the grouping of syndromes by chief-complaints due to Chinese language barriers would have effects on the outbreak detection ability, we first analyzed our data according to the ICD-9 coded syndrome groups [15] . Definitions of clinical syndromes were based on two different sources: (1) those associated with bioterrorism-related agents as announced by the Centers for Disease Control and Prevention (CDC) in the U.S.; and (2) those identified as important by the ED-SSS Advisory Committee in Taiwan, whose members include infectious disease physicians, emergency doctors, pediatricians, and epidemiologists [16] . For example, because the epidemics of enterovirus 71 caused severe fatal cases in Taiwan in the years of 1998, 2000 and 2001, the EVI syndrome group was considered as an important syndrome group locally (ICD-9 codes listed in Table 2 ) [17] . All patient information was de-identified and only aggregated data was used for data analysis. The protocol for this study was approved by the Research Ethical Committee (Institutional Review Board) of National Taiwan University. 20 Coxsackie carditis, unspecified 074. 21 Coxsackie pericarditis 074. 22 Coxsackie endocarditis 074. 23 Coxsackie myocarditis, Aseptic myocarditis of newborn 074. 3 Hand, foot, and mouth disease Vesicular stomatitis and exanthem 074. 8 Other specified diseases due to Coxsackie virus, Acute lymphonodular pharyngitis Although the ED-SSS data started transferring on March 10, 2004, we confined our analysis to data collected between April 1, 2004 (when the data became more stabilized) and March 31, 2005 . Data were organized using statistical programs to perform a descriptive analysis of the daily and weekly plots of different syndrome cases and obtain a baseline pattern for each syndrome in Taiwan. We initially generated the SQL commands for data querying and data grouping into the 11 different syndromic groups. To increase the sensitivity of this ED-SSS in monitoring regional patterns of these 11 syndrome groups, we categorized ED-SSS data by four different geographical areas (northern, central, southern and eastern Taiwan), based on major regional variations in the types of infec-tious diseases. In analyzing the seasonal patterns of ED visits, the correlation between the ILI syndrome and respiratory or asthma syndrome was assessed by the value of Pearson's coefficient (R). the Taiwan-CDC on a daily basis. The greatest challenge as began to develop this system was communication with different hospitals. Because different hospitals were using different information systems and inputting different data with various formats, it took long time to agree which variables and their data format should be collected. At the very beginning to build the ED-SSS, we had at first intended to collect information on a large set of epidemiologically useful variables, including occupation, travel history, family clustering, other exposure-related factors, and address for each patient to help detect possible zoonosis. However, such data were not collected during routine medical examination and care. Finally, we decided to capture only parameters usually collected by the hospitals during examination, intake and care.