American foulbrood (AFB, "Histolysis infectiosa perniciosa larvae apium", "Pestis americana larvae apium"), caused by the spore-forming "Paenibacillus larvae" ssp. "larvae" (formerly classified as "Bacillus larvae"), is the most widespread and destructive of the bee brood diseases.

"Paenibacillus larvae" is a rod-shaped bacterium, which is visible only under a high power microscope. Larvae up to 3 days old become infected by ingesting spores that are present in their food. Young larvae less than 24 hours old are most susceptible to infection. Spores germinate in the gut of the larva and the vegetative form of the bacteria begins to grow, taking its nourishment from the larva. Spores will not germinate in larvae over 3 days old. Infected larvae normally die after their cell is sealed. The vegetative form of the bacterium will die but not before it produces many millions of spores. Each dead larva may contain as many as 100 million spores. This disease only affects the bee larvae but is highly infectious and deadly to bee brood. Infected larvae darken and die.