There's a fair amount of difference between Nurgle and Buddha.
Buddhism is and has been the world's fastest growing religion and it has the most retention among its followers.
It would throw most of the Imperium upside down.
That said I'm fairly sure the other factions wouldn't care, and would merely take advantage and just kill all of humanity.
Without humanity Chaos would turn its full attentions upon the eldar, who would be swiftly eradicated, and then the Tau.
Chaos would die fighting the Tau and Orks. Whether this is because as the last real 'faction' that could be Chaotic, or them just being immune, I couldn't say. But suddenly you'd live in a universe that had nothing that could be corrupted to Chaos.
Only the Necrons and the Tyranid. Death and life.
Death wins, if only because death always wins. As long as there is life, there is death.
And so the Necrons go to sleep after they eradicate every last microbe in the universe, leaving it pretty cheerful, if quiet.