  This paper develops a Nash-equilibrium extension of the classic SIR model of
infectious-disease epidemiology ("Nash SIR"), endogenizing people's decisions
whether to engage in economic activity during a viral epidemic and allowing for
complementarity in social-economic activity. An equilibrium epidemic is one in
which Nash equilibrium behavior during the epidemic generates the epidemic.
There may be multiple equilibrium epidemics, in which case the epidemic
trajectory can be shaped through the coordination of expectations, in addition
to other sorts of interventions such as stay-at-home orders and accelerated
vaccine development. An algorithm is provided to compute all equilibrium
epidemics.
