[Willy]

Originally Posted by thrash

In a campaign where this is an in-context fact rather than a genre convention, it could point to a long pattern of cross-dimensional contacts in the past. Since the vectors would likely be individuals or small groups, the outbreaks would normally have been less than catastrophic and allow immunities to develop over time. Most timelines have fairly restricted in-world transportation, so diseases would be leveled across alternate versions of the same continent while not necessarily being shared between continents.

I think it´s a the other way, even fairly low tech societies, had diseases travelling via silk road, ocean going ships from continent to continent, just look at the plague, which hir the old Rome under Justinian, only to come back in medieval times. As long as enough people are there so that the deisease can spread out and still finding new victims there is no natural dying out of the disease. the so called new world also had some nasty things that moved with ease through europe, even if it was not such a killer than smallpox. and in low tech societies people only can pray but otherwise don´t do much whit every mass being a spreader event. Only if there is magical healing the game is changed.
Vaccination and the active development of antybodies and vaccines is a fairtly late TL 8 technique.

Originally Posted by zarawesome

The danger of cross-dimensional infectious diseases is already rather understated in any campaign.

Don´t we have in the Infinite worlds book the gotha worlds, with the hind that all this worlds were linked in earlier times by crosstime travel ?