For the most part, I believe they spread when the Varroa overrun a colony, the bees disperse from the hive, and enter other nearby hives looking for a new home. I'm sure they can and do live outside the hive; they had to come from somewhere, right? Just think about the days not too long ago when Varroa were here and very few beekeepers did (or could do) anything to control the populations! Have you read Wikipedia's article on Varroa Destructor? They have been found to be hitching rides on other insects to find bees! Also consider the traveling beekeeper in search of crops to pollinate or a warm place to overwinter. In my honest opinion, the most important thing WE can do is keep mite populations as low as possible (perhaps science will one day find a permanent solution); if your neighboring beekeeper does nothing to control his/her mite populations, it's going to be a constant battle. I can't think of which is worse, Varroa mites on bees, or fleas on dogs.