  Investigations of a possible connection between population density and the
propagation and magnitude of epidemics have so far led to mixed and
unconvincing results. There are three reasons for that. (i) Previous studies
did not focus on the appropriate density interval. (ii) For the density to be a
meaningful variable the population must be distributed as uniformly as
possible. If an area has towns and cities where a majority of the population is
concentrated its average density is meaningless. (iii) In the propagation of an
epidemic the initial proportion of susceptibles (that is to say persons who
have not developed an immunity) is an essential, yet usually unknown, factor.
The assumption that most of the population is susceptible holds only for new
strain of diseases.
  It will be shown that when these requirements are taken care of, the size of
epidemics is indeed closely connected with the population density. This
empirical observation comes as a welcome confirmation of the classical KMK
(Kermack-McKendrick 1927) model. Indeed, one of its key predictions is that the
size of the epidemic increases strongly (and in a non linear way) with the
initial density of susceptibles.
  An interesting consequence is that, contrary to common beliefs, in sparsely
populated territories, like Alaska, Australia or the west coast of the United
states the size of epidemics among native populations must have been limited by
the low density even for diseases for which the natives had no immunity (i.e.,
were susceptibles).
