  Using a probability of novel encounter derived from a physical model, we
augment the SIR compartmental model for disease spread. Scenarios with the same
initial trajectories and identical $R_0$ values can diverge greatly depending
on the speed at which our circles of acquaintances grow stale - leading to
order of magnitude differences in final case counts. A momentum effect arises
from variation in the mean time since infection, and this feeds back into new
infection rate and faster decline in the late stages of an outbreak. Rapid
extinction of an outbreak can occur in the early stages, but once this
opportunity is missed the effect is diminished and then, only herd immunity can
help.
