Perhaps your husband will also be interested in my theory that it’s not only popluation density, but also the use of mass transit. LA and most of the west coast travels by car, whereas the eastern seaboard and European cities are much more likely to use subways, trains, and buses. A closed box with lots of people crammed close together with limited air circulation and many commonly-touched surfaces make mass transportation an almost perfect virus incubator.
I totally agree - mass transit/ giant petri dishes. Bad enough when there’s no virus; terrible now. And still they keep them open.