The bad news for those who like CSI-style endings is that it looks like there's no one culprit. Evidently a range of stressors are undermining the bees' ability to fend off disease.
One of these stressors is especially interesting to wildlife-friendly gardeners, however: Namely, the bees are apparently suffering from a flower shortage. To quote the article:
"We and other experts also suspected that the bees' natural defenses might be undermined by poor nutrition. Honeybees and wild pollinators, too[,] no longer have the same number or variety of flowers available to them because we humans have tried to 'neaten' our environments. We have, for example, planted huge expanses of crops without weedy, flower-filled borders or fencerows. We maintain large green lawns free of any 'weeds' such as clover or dandelions. Even our roadsides and parks reflect our desire to keep things neat and weed-free. But to bees and other pollinators, green lawns look like deserts. The diets of honeybees that pollinate large acreages of one crop may lack important nutrients, compared with those of pollinators that feed from multiple sources, as would be typical of the natural environment." (Emphasis added.)
So here's my new motto, something to remember the next time you're giving yourself a hard time for having a "messy" garden:
Let yourself be messy. The bees (and a lot of other critters) will live better if you do.