Fig. 3. Time evolution of the density of infected persons I(x, y, t) for a local source of infected persons (airport).

The time evolution is shown for strong interactions with C_si = 3C_sd = −30 and different influx strengths I_source. Initially, one observes radial spreading (transient regime). Later, a steady state is reached, which is stationary for a small influx. A larger influx leads to a periodic regime (I(x, y, t) = I(x, y, t + nτ) with $n\in \mathbb{N}$ and period duration τ ≈ 16), involving the formation of a complex oscillating structure.