The aim of our study was to develop a scoring system to predict whether diarrhea is of a bacterial origin and whether the diarrheal patients constitute a potential source of infection to others. Adults with acute diarrhea (n=424) were enrolled in the study. Logistic regression and standard regression coefficients were used to formulate the Early Warning Infectivity Score System for Adults with Acute Bacterial Diarrhea (EWIS-ABD). Four risk factors were identified by logistic regression, including body temperature (P<0.01), abdominal pain (P<0.01), leukocyte count in stool (P<0.01), and unclean dietary history (P<0.01). EWIS-ABD was thus developed, in which the value >= 5 points was set as an indicator of bacterial diarrhea. The incidence of bacterial diarrhea increased along with the elevated score. EWIS-ABD was more specific for bacterial diarrhea than for viral diarrhea. The accuracy and reliability of EWIS-ABD was high by prospective validation in 478 patients with acute diarrhea.