  Regional quarantine policies, in which a portion of a population surrounding
infections are locked down, are an important tool to contain disease. However,
jurisdictional governments -- such as cities, counties, states, and countries
-- act with minimal coordination across borders. We show that a regional
quarantine policy's effectiveness depends upon whether (i) the network of
interactions satisfies a balanced-growth condition, (ii) infections have a
short delay in detection, and (iii) the government has control over and
knowledge of the necessary parts of the network (no leakage of behaviors). As
these conditions generally fail to be satisfied, especially when interactions
cross borders, we show that substantial improvements are possible if
governments are outward-looking and proactive: triggering quarantines in
reaction to neighbors' infection rates, in some cases even before infections
are detected internally. We also show that even a few lax governments -- those
that wait for nontrivial internal infection rates before quarantining -- impose
substantial costs on the whole system. Our results illustrate the importance of
understanding contagion across policy borders and offer a starting point in
designing proactive policies for decentralized jurisdictions.
