Real life shows us that diseases developed in separate habitats (in casu Old World and New World) can be of significantly different number and virulence. The Amerindians got several virulent diseases from Europe; the Europeans got one, and there's even a theory that syphilis didn't come from the New World at all. So I don't see anything unlikely about Earth having developed more and worse diseases than any of the Ancients' terrariums had.
Hans
Agreed Hans. And, I think that the Vilani were wise quite wise in worlds that had a fecund of micro-organisms (i.e. unfriendly bacteria) and humaniti, they simply quarantined and did not allow the cultures to advance past TL 3.
Indeed, syphilis did not come from the New World - not that the new world was sterile but just cut off as you rightly said.
However, I do suspect something more sinister about the Plague of Duskir - perhaps, it was a tactic to weaken Vilani resolve and superiority. Kindly, Medicins sans Frontiers fanning through the Stars like missionaries would do more to win over the hearts of lower order Vilani castes than orbital bombardment especially if indeed the transmission of the virus was through the food - thus breaking up the Vilani social order with Shugaii (sp) being replaced with a new caste - the medic. For we think that there are hundreds of ways of doing medicine but if only one caste has the healing properties (like in the case of Chinese traditional herbal medicine) - the authority of the Shugaii replaced with something equally scientific but different would have shattered the Caste system along with the reforms of promoting free market capitalism.