Just like in Ryhe'n fall of Civilizations Mod, spreading plague would be interesting and also usefull to improve the turn speed 
The actual plague events are just scripted and not dynamic like the ryhe's one, i dunno if it's a realistic idea, but would be great to implement!

The actual plague events are just scripted and not dynamic like the ryhe's one, i dunno if it's a realistic idea, but would be great to implement!