The "deterministic simple epidemic" unmasked.

We derive a simple but surprising connection between the "deterministic epidemic with recovery" and the spatial birth-and-death process, and discuss its implications for the use of nonlinear differential equation models for the spatial spread of epidemics. This paper consists of a manuscript dating back to 1977, which has been widely referred to in the literature but not previously published, and its more recent extension to include recovery/death in the processes considered, with some additional comments. To aid reference, we retain the original title.