Bob said:

> Unknown is why the bees leave the hive to die.

"Leaving the hive to die" is actually fairly common, 
and is considered a very basic and important part of 
the "colony immune system", not to be confused with 
the immune system of a bee.

The difference with CCD is only:

a) The quick spread of the disease pathogen within the hive
 
b) The lack of symptoms among the remaining bees

c) The virulence of the pathogen, proven by the apparently
   healthy hive with a large brood area degrading to a 
   "depopulated" hive with little more than the brood area
   left.  

d) The lack of clear evidence (so far) as to the specific
   agents at work.

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