  In this paper, we study how interactions between populations impact epidemic
spread. We extend the classical SEIR model to include both integration-based
disease transmission simulation and population flow. Our model differs from
existing ones by having a more detailed representation of travel patterns,
without losing tractability. This allows us to study the epidemic consequence
of inter-regional travel with high fidelity. In particular, we define
\emph{travel cadence} as a two-dimensional measure of inter-regional travel,
and show that both dimensions modulate epidemic spread. This technical insight
leads to policy recommendations, pointing to a family of simple policy
trajectories that can effectively curb epidemic spread while maintaining a
basic level of mobility.
