Colony Collapse Disorder or "CCD" is a condition in which the bulk of the bee colony seems to disappear. A beekeeper will have left an apiary with everything seemingly fine and return only to find the hives either empty or with only a handful of bees left.
First noticed in the United States in 2006 by Dave Hackenberg, a Pennsylvania beekeeper who over wintered his bees in Florida.
There are many theories as to why this has occured but the current consensus is that it is caused by a combination of new pesticides in the neonicotinoid class (clothianidin and imidacloprid being the ones most frequently mentioned).
These pesticides effect the nervous system of the bees and it is believed that after exposure the bees can no longer navagate back to the hive and die in the field.
Since it takes the nectar of 2 million flowers to produce a pound of honey it is no wonder that worker bees are over exposed to these pesticides.