This subject was tackled by Ingemar Fries who delivers an excellent talk. He first covered the epidemiology of this communicable disease, describing how it spreads, often with the assistance of beekeepers!
He discussed horizontal versus vertical transmission, the former occurring when it doesn’t matter to the organism if it kills its host as there’s always another one nearby. The habit of beekeepers of having lots of colonies in apiaries instead of spacing hives well apart assists horizontal transmission.
With vertical transmission on the other hand, the bug depends for its own survival on that of the host and so those which are more benign/less virulent have a better chance of survival long term.
The present UK policy of burning diseased colonies eliminates the most virulent strains of AFB and the most susceptible colonies. There are colonies that aren’t troubled by AFB and Roger Morse once told me that he had colonies which he had found impossible to infect with it!
Did you know that humans can get AFB? It seems that some druggies inject themselves with their poison mixed with honey. If the honey has AFB then it can multiply in the body, causing septicaemia. Now that honey is coming increasingly into use, once more, as a wound dressing, it might be sensible only to use honey from known and trusted sources for this purpose.