Coupled radiation and fluid flow problems are pertinent in many applications including nuclear reactor physics, atmospheric flows, photon-transport through clouds, and thermal heat transfer. Here a new model is presented that can simulate general coupled radiation-fluids problems, and the focus of this talk will centre on the application of the model for the analysis and optimisation of disease inactivation in air and water using Ultraviolet-C (UVC) devices. Quantifying the efficacy of inactivation of air \& water-borne disease through UVC is complicated. Each situation is unique, with its own atmospheric/flow conditions with spatially varying UVC intensities. Assessing the transport and the UVC-inactivation of disease is therefore a multi-physics (radiation-fluids) problem, perhaps best suited to computational modelling. We present a new high-fidelity model for simulating coupled radiation transport and CFD for such a purpose \cite{buchan1}. We first present our recent (and first such) analysis in using 222nm UVC to disinfect air, potentially safely in the presence of people. Through this high fidelity modelling we showed the approach can be used in combination with ventilation to disinfect rooms up to 85\% more effectively, than ventilation alone, but then also through mixed experiment-modelling reveal the approach to be more effective than first realised \cite{buchan2,welch}. We then present the reapplication of the model for the simulation and analysis for UVC-reactors used in water disinfection. In addition to the applications this talk will present some more unique numerical methods that enable the fast but accurate simulations including the constructions of specialised reduced order models for these problems.