An overview of the processes and FOSS tools used to model and implement medical decision support can be found in, Free and Open Source Enabling Technologies for Patient-Centric, Guideline-Based Clinical Decision Support: A Survey by T.-Y. Leong, et al in the IMIA Yearbook of Medical Informatics, 2007.

LinuxMedNews is mentioned as one of several good places to find Open Source healthcare software! See note 19.