  We discuss the effects of spatial interference between two infectious
hotspots as a function of the mobility of individuals (wind speed) between the
two and their relative degree of infectivity. As long as the upstream hotspot
is less contagious than the downstream one, increasing the wind speed leads to
a monotonic decrease of the infection peak in the downstream hotspot. Once the
upstream hotspot becomes about between twice and five times more infectious
than the downstream one, an optimal wind speed emerges, whereby a local minimum
peak intensity is attained in the downstream hotspot, along with a local
maximum beyond which the beneficial effect of the wind is restored. Since this
non-monotonic trend is reminiscent of the equation of state of non-ideal f
luids, we dub the above phenomena epidemic condensation. When the relative
infectivity of the upstream hotspot exceeds about a factor five, the beneficial
effect of the wind above the optimal speed is completely lost: any wind speed
above the optimal one leads to a higher infection peak. It is also found that
spatial correlation between the two hotspots decay much more slowly than their
inverse distance. It is hoped that the above findings may offer a qualitative
clue for optimal confinement policies between different cities and urban
agglomerates.
