Each of the infectious diseases that GUARDIAN monitors is modeled in GUARDIAN's knowledge base as a SNBN (1) of diagnostic features. Each model is developed through extensive literature review by infectious disease experts and encoded as a standardized probabilistic SNBN (2) . GUARDIAN uses these networks to rapidly assess each patient whose data are presented to the system. General model fitness is evaluated using 10-fold cross-validation. The overall, threshold-independent, model quality is determined through analysis of the receiver operating characteristic (ROC) curve (3) of each model using de-identified emergency department patients for the true negatives. To find the thresholds for each relative risk category, we use Gaussian decomposition (4) over the probability distribution of known negatives; known positives of other diseases; and known positives of confirmed, suspected and probable cases of the disease of interest.