Fig. 1.

Dynamics of infection in a heterogeneous host population comprising unfavorable (U, gray squares) and favorable (F, white squares) sites. The state of an epidemic is shown for two time points, with solid circles indicating infected hosts and empty circles indicating susceptible hosts. The arrows represent potential transmission routes between infected and neighboring susceptible sites. The figure shows how the localized conditions change with time and how the disease load depends on the relative strength of each of the four possible transmission rates β[., .]. (Left) Total infective challenge on F is 4β[F, F] and 3β[F, U] on U. (Right) As more hosts become infected, this challenge changes to 3β[F, F] + β[U, F] on F and 2β[F, U] + 2β[U, U] on U. Even though the number of transmission pathways has increased from seven to eight, the combined effective challenge may be greater or less than in Left, depending on the relative strength of each of the four transmission rates.