A little history is in order:

Excerpts:

Disappearing Disease
by James E. Tew

Some History

The condition was first described in 1915 and was called Disappearing
Disease because the disease was self-limiting and disappeared. Through the
years, that name has increasingly been broadened to describe any mysterious
instance where adult bees disappear - not the disease. Confusing isn t it?
If the bees have disappeared, then the disease is gone, too. Right? From
1915 until this time, no single pathogen has even been isolated. Other
possible names for the ailment are: Spring Dwindling, Fall Dwindling, May
Disease, and Autumn Collapse. The Isle of Wight Disease, caused by tracheal
mites, has many similarities to Disappearing Disease. The reported symptoms
are broad and indistinct appearing to be a collage of characteristics.

In 1915, after a particularly wet Spring, significant colony losses were
reported. One beekeeper lost 400 hives. The problem was noted in multiple
states from Florida to California. Hives came out the Winter in good shape,
but adult bees began to vanish at the beginning of the Spring nectar flow.
In afflicted apiaries, at best, honey crops were reduced. At worst, colonies
were essentially emptied of adult bees. During subsequent years, now and
then, reports were posted presenting Disappearing Disease as the cause of
occasional colony losses.

Characteristics of Disappearing Disease

Adult bee loss with no accumulation at the hive entrance.
Adult bee loss after a cool damp Spring - though losses have also been
reported in the Summer and Autumn.
Queens are the last hive individuals to be affected.
Pollen and honey stores are strangely normal.
A disproportionate brood/adult bee ratio.

etc

see: http://www.orsba.org/htdocs/download/Dtew.htm

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