INTRODUCTION Differences in epidemic severity arise from differences in individual susceptibility (i.e. propensity to acquire infection), infectivity (i.e. propensity to pass on infection to others) and recoverability (including true recovery but in some cases death). Identifying genetic and non-genetic factors affecting these three traits is of crucial importance, because it opens up the possibility for selective breeding or control measures aimed at reducing the impact and prevalence of disease. Recently it has been shown to be possible to estimate these factors from longitudinal epidemiological data (e.g. infection and/or recovery times of individuals or diagnostic test results) using Bayesian statistical methods implemented in the software tool SIRE. PAC SOFTWARE The "prediction accuracy calculator" (PAC) is a tool that can be used to help design disease transmission experiments to maximise the information they provides regarding variation in traits arising from a given effect. This tool accompanies a soon to be published paper. DOWNLOAD Download the file PAC.zip, unzip, and click on "pac.html" to open the software in a browser window.