This chapter discusses in more detail the concepts of contagion and contagious agents in network and computer simulation studies against the backdrop of epidemic response systems. It examines that the understanding of contagion as a cross-section of biology and computer technology – especially in Agent-based models (ABMs) – entails at least two major consequences. Contrariwise, and building on a research tradition of functionalist approaches to animal collectives, recent ABM in biological research has shown that information is distributed through the constantly changing and moving collective by a large number of parallel, individually distributed, and local actions and reactions. Cellular-automata (CA) models for disease spread improve upon these models and allow for spatial dimensions and discontinuities. However, the geometry of CA highly oversimplifies the spatial reality of the real world and thus of disease propagation processes. The application of ABM thus transforms the modes of describing and of acknowledging dynamic systems