mmadsen said:
It depends on the time frame you're looking at. In the short term, population centers are much more vulnerable to epidemics -- particularly if they bring together multiple species that can spread diseases back and forth, e.g. pigs and humans.
In the long term, urban populations develop immunities to the various diseases that have passed through, while distant hunter-gatherer populations do not.
Thus, when the Europeans started arriving in the New World, locals died in droves.
Not to nitpick but Tenochtitlan, as the largest city in the hemisphere (with a population of about 200,000) had amongst the highest death rates from smallpox. Densely-populated areas of the Mexico Valley and Andes had the worst epidemic disease at contact of almost anywhere.
Fortunately, the history of the Americas nevertheless supports your general contention. The Andes and Mexico Valley have had, amongst the best demographic recovery of any indigenous populations in the New World, largely because, after a 90% drop in population in a century and a half, the population density remained sufficient to sustain a productive society whereas lower death rates were able to annihilate societies that lacked the characteristics of Mesoamerican urban societies.
Also, on this front, the main thing that conditions whether a society will suffer what Crosby called "virgin soil epidemics" is not urban life; rather, it is trade dependence. African societies on the Indian rim had high levels of resistance to Eurasian disease than did less trade-dependent interior societies, despite the fact that population densities remained low and settlement patterns remained rural.
Ultimately, the societies that have the best disease resistance tend to be migratory pastoralists, not city dwellers. In migratory pastoralist societies, contagion exposure is not as bottlenecked through a small number of trading specialists but is more evenly distributed through society.
EDIT: Recent science seems to indicate that it is this ranging and trading, not the zoonotic explanation that accounts for this.