Every third bite of food you take, thank a bee or other pollinator. – E. O. Wilson
Colony collapse disorder (CCD) is the phenomenon that occurs when the majority of worker bees in a colony disappear and leave behind a queen, plenty of food and a few nurse bees to care for the remaining immature bees and the queen. In the six years leading up to 2013, more than 10 million beehives were lost, often to CCD, nearly twice the normal rate of loss. 
Honey bees enable the production of at least 90 commercially grown crops in North America. Globally, 87 of the leading 115 food crops evaluated are dependent on animal pollinators (which include honey bees), contributing 35% of global food production. Imagine a world without these magnificent pollinators. 
In early 2007, abnormally high die-offs (30–70% of hives) of European honey bee colonies occurred in North America; such a decline seems unprecedented in recent history. it is unclear whether this is simply an accelerated phase of the general decline due to more adverse conditions in 2006, or a novel phenomenon. CCD is unique due to the lack of evidence as to what causes the sudden die off of adult worker bees, as well as there being few to no dead bees found around the hive. 
Research has so far failed to determine what causes it, but the weight of evidence is tentatively leaning towards CCD being a syndrome rather than a disease, as it seems to be caused by a combination of various contributing factors rather than a single pathogen or poison. However, in April 2013, after a report was released by the European Food Safety Authority identifying the significant risks of the class of pesticides called neonicotinoids (neonics for short), the European Union called for a two-year restriction on neonicotinoid pesticides.
There have been no effective preventative measures against CCD suggested to date.
References and Additional Information
 Earth Justice: The Case of the Vanishing Bees
 USDA: ARS Honey Bee Health and Colony Collapse Disorder
 NRDC: The Buzz About Colony Collapse Disorder
 Forbes: The Cause Of Colony Collapse Disorder, Disappearing Bees Becoming More Clear
 Bulletin of Insectology: Sub-lethal exposure to neonicotinoids impaired honey bees winterization before proceeding to colony collapse disorder
 EPA: Colony Collapse Disorder
 Native Plants and Wildlife Gardens: What’s All the Fuss About Neonicotinoids?
 Wikipedia: Colony Collapse Disorder