This document discusses open source software and its applications in public health. It describes open source as emphasizing freedom to use, modify, and distribute source code without necessarily being free of cost. Several open source license models are mentioned. The document then outlines benefits and challenges of open source software, and provides examples of open source applications that are used in public health, including Biocaster for infectious disease detection, TranStat for estimating disease transmission, and Sahana for disaster management.