To date, most models of viral infections have focused exclusively on modeling either the intracellular level or the extracellular level. To more realistically model these infections, we propose incorporating both levels of information into the description. One way of performing this task in a deterministic setting is to derive cell population balances from the equation of continuity. We apply such a balance to obtain a two-level model of a viral infection. We then use numerical simulation to demonstrate both cell culture and in vivo responses given a variety of experimental conditions. We compare these responses to those obtained from applying other commonly used models. The results demonstrate that, in contrast to commonly used models, the cell population balance provides a more intuitive and flexible modeling framework for incorporating both the intracellular and extracellular events occurring during viral infections. This improved capability to represent the trends in the biological measurements of interest offers a more systematic and quantitative understanding of how viral infections propagate and how to best control this propagation. (c) 2004 Elsevier Ltd. All rights reserved.