Clinicians and patients typically experience difficulty with the conditional probability reasoning (Bayes Theorem) required to make inferences about health states on the basis of diagnostic test results. This problem will grow in importance as we move into the era of personalized medicine where an increasing supply of imprecise diagnostic tests meets an increasing demand to use such tests on the part of intelligent but statistically innumerate clinicians and patients. We describe a user friendly, interactive, graphical software interface for calculating, visualizing, and communicating accurate inferences about uncertain health states when diagnostic information (test sensitivity and specificity, and health state prevalence) is relatively imprecise and ambiguous in its application to a specific patient. The software is free, open-source, and runs on all popular PC operating systems (Windows, Mac, Linux)