- Faction blahblahblah settlement needs help repealing the flood:
If player does not get involved, the battle is automatically simulated favoring the side with the strongest force. If the flood wins, the settlement can go dark where the flood takes over. Anything withing that settlement range will become the next victims randomly. Including players settlement/camps/outposts if withing the radius.

- Faction blahblahblah needs help attacking infested settlement:
Same thing applies as the first suggested event.

You don't have to add the flood unit moving around the world map. Just add them as events and settlements switching to infected settlements where other events triggers withing infected settlement range.