  During a pandemic, there are conflicting demands arising from public health
and economic cost. Lockdowns are a common way of containing infections, but
they adversely affect the economy. We study the question of how to minimise the
economic damage of a lockdown while still containing infections. Our analysis
is based on the SIR model, which we analyse using a clock set by the virus.
This use of the "virus time" permits a clean mathematical formulation of our
problem. We optimise the economic cost for a fixed health cost and arrive at a
strategy for navigating the pandemic. This involves adjusting the level of
lockdowns in a controlled manner so as to minimise the economic cost.
