So there must be a third way plagues can happen that is totall uncontrollable by modders:

1. "script" plague by descr_events.txt historical events file - need to specify place where it occurs
2. cause "random" plagues by descr_disasters.txt file - places where they occur need to be specified by region or coordinates or terrain
For EB, this group is currently "off" -
event plague
frequency 2
winter false
summer false
min_scale 2
max_scale 5
3. a third uncontrollable by modders way that is hardcoded to cause plagues in towns with overpopulation and little to no health improvements