Yeah, that's right, blame me Charles.
Marches weren't even available at that point, so you would not have built a march, and I seriously doubt you had the gold to build even an outpost, if you did you would have built one in another region, such as the one now claimed by the Mist Elves.
EDIT 4: And if you had sent your units into the area they would have probably all caught the disease, turned on your now completely unprotected cities, and with the victims they took there the problem would have been even worse then it is now. You would have only made things worse. Looking at it that way, the Bannor actually held the undead attack off by preventing your troops from catching the disease, and if it wasn't for our actions the situation would be far, far worse.
When I protected the Dove-Yellow it was because they were a threatened race and as the Bannor, I felt it was the duty of my people to protect them from the heretical rule of the Grigori. When they became diseased (and you vassalised to me, it was more or less the same time), I withdrew all support from them, at this point you could have built your outpost if you cared so much. I had more important things on my mind and the disease didn't seem too much of a threat, the only example I had of it spreading was to my diplomats and the disease was very well contained. I could not have known about the extent of the problem we would face.
EDIT: Also, how could I have protected you from that? If I had tried I would have made matters far worse, there'd have been even more zombies and your city would have been even more trashed. If you are talking about tribute when you mention 50 gold, then you should know that at most you sent 24 gold, but it was probably more like 18.
EDIT 2: At the time of the excecution of the fists we had an joint invasion planned against you(EDIT 3, why else would we have killed your best unit?), you should feel grateful that I pulled out of the campaign, causing it to be abandoned, otherwise you woldn't even be in the game right now.