A pandemic doesn't come about from diseases that evolve within the same population, for at least 2 reasons. The first is that the disease co-evolves with the population, and being a pandemic is not a good survival strategy. A dead host cannot continue to spread copies of a disease. Secondly, large parts of the population would have partial immunity from infections leading up to a would be 'pandemic' strain, causing it to not be a pandemic at all.

So to answer a later point, lots of disconnected hamlets are far more likely to breed pandemics, since a disease can evolve to be highly virulent in one population, and suddenly get dumped on a population with no natural (through antibodies or natural selection) resistance to it.