>     Thanks for the answers to my query. The only thing is,
> both the main factors mentioned so far - cell size and colony
> density - also applied in the UK, and we didn't have an
> epidemic.

My thoughts are that the strain of bees involved (Italian) with it's
tendency to rob may have been part of the problem.

Also American Foul Brood is misnamed. I believe it came from Europe
(European Foul Brood from the United States). I believe the names came
from where they started causing problems. The European bee population
may have already developed a resistant trait to the bacteria involved.
When the bacteria arrived in the United States, with its relatively
closed gene pool, things got out of hand.

Just my thoughts.