NATO Allied Medical Publication 7.5 (AMedP-7.5), NATO Planning Guide for the Estimation of Chemical, Biological, Radiological and Nuclear (CBRN) Casualties, includes a contagious disease model for estimating the number of casualties resulting from an attack with a contagious biological agent. The current document describes a new contagious disease model developed by the Institute for Defense Analyses that overcomes certain known limitations of the AMedP-7.5 contagious disease model and could replace the existing model in future versions of AMedP-7.5. In particular, the new model relies on aggregated transmission data from multiple historical outbreaks of disease rather than transmission rates fitted to a single historical outbreak per disease. It includes the capability to model isolation of infectious individuals within a user-specified time after symptom onset. Lastly, the new model overcomes the simplifying assumption that individuals transition between cohorts at a constant rate equal to one divided by the mean time in a cohort. Instead, the new model incorporates convolutions to model the transition of individuals according to time distributions derived from extensive literature reviews.