When I was younger CCD had a totally different meaning than the popular Bible study all of my friends attended, I was taught about CCD from a young age when my father's hives were abandoned for no apparent reason. One of the most puzzling things about CCD is that all adult bees leave the hive only to leave healthy brood, or eggs and larva, and untouched honey stores.
The cause of CCD is unknown and scientists work hard to find a cause. Although the investigation is ongoing, there a serval leading candidates.
Parasite and Pathogens:
One of the first things scientists investigated was the parasites, like small hive beetle, that invade the hive and may carry the cause of CCD. While it is not likely parasites are the cause of CCD they may make the hive more susceptible to it. Scientists biggest focus is on the varroa mites because they take a huge toll on the honey bee's health. If the pests themselves are not the cause, it may be the use of chemicals to rid the pests that affecting the bees as well.
Pollution:
Not an unlikely cause, human industrialization may be taking a toll on honey bees. The air is polluted by many chemicals and honey bees may be inhaling it. Also, water is treated or contains large amounts of chemical runoff. The possibilities for toxic exposure make pinpointing a definitive cause difficult, but this theory requires more attention from scientists.
Lack of Diversity:
Honey beekeepers, especially commercial, depend on breeders to supply them with queen bees in order to expand their hives. All of these bees descend from about 100 different breeders. This lack of genetic diversity makes bees more susceptible to environmental factors.
Cellphones:
There is a theory that cellphone towers and the electromagnetic radiation they admit may be to blame. Despite this widely believed theory, after investigating the behavior of honeybees when exposed to close range electromagnetic radiation, no evidence to support this theory was found.
Migrating Hives:
Commercial beekeepers rent their hives to farmers to help pollinate crops. Hives are moved, stacked, covered, and driven thousands of miles every few weeks. For honeybees, orientation to their hive is vital to life. Small-scale beekeepers see backlash when they relocate the hive just a few feet from its original location. The stress alone may cause CCD as well as the chances of parasites and disease being spread much easier through relocation. My personal thought on this theory is that the physical relocation probably does not cause CCD due to the fact that small-scale or commercial beekeepers who do not move hives experience CCD as well.
Although these are just a few possible theories and there are many other reasons being investigated scientists are working hard to get to the root of the problem. Hopefully one day CCD will an issue of the past. Thanks to modern technology, every day we're one step closer to finding answers.