Peter,

The larvae that die of EFB do so before they defecate and are removed by the
undertakers intact without spreading their gut contents of bacteria.  Those
that live to pupate do defecate.  The first duty of a young bee after
emergence is to clean up cells using her tongue.  Her next duty is to feed
the next generation of babies, again using her bacteria laden tongue.  This
is the main way of passing the bacteria within the hive.  Between hives I
would guess drifting and robbing if left to nature, but the beekeeper is
probably the main cause if the bees have one.

There is much that is not known about EFB, particularly the life span of the
bacteria.  They do not form spores as AFB bacteria do, and this is why I
asked the question about fumigation of combs, but clearly they can live long
enough in the hive to survive long broodless periods without the protection
of a spores coat.

Chris