A software ecosystem consists of a shared software platform, where internal and external developers interact to deliver software to customers. Software ecosystems are reliant on external developers creating innovation for the software ecosystem to thrive. However, we lack knowledge about how to attract external developers and how to enable them to innovate. Research has shown that software reuse can increase productivity. This thesis explores how external developers reuse software in a public software ecosystem, where resources may be scarce and the aim is not revenue growth. The following research question is asked: What are some factors that characterize software reuse by external developers in a public good software ecosystem? The Health Information Systems Programme (HISP) is a global network to strengthen health information systems. One of their most important contributions is District Health Information System 2 (DHIS2). Through the use of an Application Programming Interface (API), developers can extend DHIS2 by creating web applications. HISP India is one of the nodes in the HISP network which implements DHIS2. In an Action Case research project, I have participated as a developer in the HISP India team to implement the AMR Surveillance System based on DHIS2. I have created software packages and other resources aimed at aiding the development of DHIS2 applications. This thesis contributes to software ecosystem literature through factors that affect software reuse by external developers in public good software ecosystems. The research has also lead to practical contributions including DHIS2 applications and software packages to HISP India and the HISP network.