i'm not a scientist or anything, but my common sense seems to tell me that if i have hygienic bees that groom one another and will tear out brood and eject it from the hive when it has mites in the cell with it, that the hive would be better off by getting rid of those mites before their offspring emerges. i never had to worry about these things before the winter of 95/96, when i was only left with only four out of thirty hives. now i do have to to consider mites and how they effect my beekeeping. my first and foremost line of defense is keeping hygienic bees. as you mentioned, bees can live with a small mite load. and yes, mites do transmit a number of viruses. now if you keep bees that you have to treat because of heavy mite loads, doesn't it make sense that more of your bees have viruses due to that heavy mite load than a hive that can handle mites on their own and don't have to be treated? i don't see why more people don't try to select from the better bees in their own apiaries. they should raise some queens from their better hives and requeen the the ones that are susceptible to disease, aren't hygienic, or just plain lousy. everyone will rush to requeen a hot hive, but won't even take a little time to test their hives to see which ones are the most hygienic. you can trust me when i tell you that you will enjoy keeping bees much more when you don't have to constantly worry having varoasis all the time.