A significant fraction of today's scientific software has grown through accretion, though there is a small number of computational science codes that have undergone a design process. Among the many benefits of a well designed code is that it can expand to serve communities beyond the ones it was designed for. Thus research communities that have similar computational requirements can symbiotically improve the process of computation-based research for each other. I will present a case study of such symbiosis among communities with FLASH, a simulation code that was designed and developed for simulating thermonuclear runaways such as novae and Type Ia supernovae in astrophysics. Designed to be modular and extensible, users from several diverse research areas have added capabilities to it and adapted it for their own communities. Examples include cosmology, high energy density physics, core-collapse supernovae, star formation, fluid-structure interactions and chemical combustion. In this presentation I will give a summary of the design features that facilitated the expansion, quantify the effort needed to expand into some of the above-mentioned fields, and also the effort saved by these communities by reusing FLASH's infrastructure.