Wow, never seen it spread that far. Awesome.
Right after that it began the flu began to leave the eastern continent, and then Tamriel.
And the best thing : It screwed my campain. Since I'm stupid, after I thought I was strong enough, I waged war against the three independant kingdoms left in Skyrim (the Rift, Solitude, and Windhelm). However, several provinces were still affected by the flu, and so I had to force several assaults even though it depleted my armies. I could not finish the invasions, I still got enough to be the High King of Skyrim, but my vassals demanded the Elective Succession (and since my armies were basically wiped out from my wars, I could not say "Nope, screw you"). Of course, they immediatly decided that I was not worthy of keeping the High Kingdom, and were rooting for some random Jarl. I managed to stop the bleeding by giving everything except my core demesne (Whiterun) and the territories I had near Riften and Winterhold (I did not have the duchy, so my vassals would not hate me for the votes I had - and I learned that it triggers the ambition malus, so when I had two duchies some of my vassals immediatly got a 65 opinion malus). Everything else was given to Jarls, according to the de Jure map (which was good, nice, and beautiful). However it was not enough, I lost the election when my character died, I decided to try to get back the title (since it's Elective any vassal can decide to make a faction to take the title), but my ass got kicked and I lost everything.
It was really amazing to see that a plague, combined with some really poor decisions, finally made me lose the game. I learned a lot.
Oh, and Valenwood was infected when the plague started to recess, and, I think according to the lore ?, Argonia was left alone (if I remember correctly, it allowed the Argonians to waged several campains against the Dunmer, a small revenge for the several centuries of raids and slavery, and some people accused the Argonians from being the ones behind it, right ?)