it also explains why your body fights the virus with a fever. also, it may be pointed out that the plague was during the little ice age, but i had always thought that had more to do more with people in close quarters to the rats that harbored the fleas carrying the disease. (funny how i'm suddenly reminded of the song "there's a hole in the bottom of the sea".)
unfortunatly other disease, much worse than the flu, prefer the warmer climate (malaria for one).