In addition to the other ideas for risk and spread:
-Chance for spread to other (known) empires, increased by level of treaty and percentage of trade. Of course plagues can affect more than one species--look at Star Trek! Of course, then 95% of all alien species are misplaced humans, too.
-Increased risk of plague for building plague bombs (risk affects commissioning planet)
-Increased risk of plague for researching levels in Biology, Biological Weapons, or Medical Technology (risk per research facility, affecting those planets). Of course, this isn't likely to happen, because of the ability to mod these tech areas; but it would still be cool.
Slightly OT, but it would be cool to mod tech areas so there was a risk to researching them. It could function somewhat like the current events/intel model, including fields for risk percent, risk type, and effect amount. Why not go one step further and create a SideEffects.txt with component and facility family numbers, too, to assign risks to building those items. SEV, maybe.
