European foul brood is a disease which affects developing larvae.
It’s caused by a bacteria known as Melissococcus plutonius which infects pollen stores and young bees. As the bacteria establishes itself in the guts of the larva, it starves it’s host of nutrition causing death. The larvae die off into sacks of goo, and as they are broken down to refeed to other young, the bacteria is further spread to other larvae.
Large healthy colonies will fend off EFB more readily than others as hygienic behaviors and spreading of bacterial loads across much more larvae cause more difficulty for M. plutonius to establish itself within the hive.
pictures, with description of what the picture shows (and how it’s different from other diseases with similar symptoms (think EFB vs sacbrood))
Treatment options vary, but depending on your location, this might be a notifiable disease that requires testing and immediate treatment.
Usually treated with antibiotics, or colony manipulations. Some jurisdictions will no longer prescribe antibiotics for EFB because the manipulations required to treat it are straight forward and easy to perform.
As with all diseases that cause brood die offs, this may look similar to sacbrood.