Usually, it’s been the other way around. Urbanites live in close proximity which facilitates transmission, they may or may not have good sanitation systems or quarantine practices, and new diseases have a great number of hosts to spread to as well as multiple vector and reservoir species that the newcomer might be compatible with. Epidemics/outbreaks usually happen when something moderately adapted to a population makes it into a new territory. A strain of smallpox from densely populated India; black plague from China; our annual flu strains from bird migrations mixing with vast herds of animals kept by large numbers of humans mixing in one big-assed petri dish.
The danger with invading or exploring new territories is that you might run into a disease that the population there is resistant to through long exposure, but is novel to your immune system. These tend to burn through exposed populations quite quickly, but don’t usually produce epidemics since the exposed individuals either die or recover before they go back to their home territory, and the locals are resistant.
This is the pattern with most of Africa, where diseases co-evolved with the population. Humans who encounter these viruses for the first time are screwed because the things are already well adapted to circumventing human immune systems, but the newly-exposed people often don’t have any immunity at all.
Hemorrhagic fevers like Ebola or Marburg are famous because they kill the shit out of almost everyone exposed to them. There are, apparently, some few people in a very small area that show a level of immunity to these diseases, but researchers still haven’t figured out what the non-human reservoir species is. The reason they haven’t spread to a large population is that they both kill very quickly. This is usually a hallmark of a newly-introduced disease. Once multiple exposures happen, and a higher level of base immunity is developed, the virulence typically drops. Modern high-speed transportation is what makes Ebola and similar Virus X-type diseases scary.
There are some indications that virulence drops through both mutation in the virus, alongside adaptations the exposed species makes to inhibit the invader. Winning the lottery from a virus’s standpoint is to be highly infective, but not kill the host quickly, so as to spread to as large a population as possible. Rhinoviruses (common cold) and herpes are great examples of well-adapted, highly infectious, but relatively benign viruses. They’ve probably been around us for a long, long time.
In the Americas, there seem to have been very few viral and bacterial diseases that made it through the migration, and the largely hunter-gatherer lifestyle kept any encountered diseases localized. There were very few plagues until higher concentrations of people developed, which was relatively late in human history. Syphilis probably evolved in one of the more densely-populated centers of the Americas ([url=]this says the New Mexican early agricultural Mogollan culture). Aside from this notable exception, and some nasties later settlers ran into mostly near waterways, the native North Americans don’t seem to have encountered many diseases at all. The diseases they were exposed to by the Europeans were old nasty things that had been endemic in human populations since the very beginning of civilization, which is why there were die-off rates upwards of 90% across most of the continent.
On the other hand, procrastinators will never get anything done.