American Foul Brood (AFB) and European Foul Brood (EFB) are two serious bacterial diseases which affect the honeybee. They are both highly contagious and can rapidly lead to the death of a colony. Signs of the diseases include discoloured and irregular brood (shown below). You will also start to see a dramatic decrease in hive activity as these diseases will kill off most of the larvae.
Although they exhibit similar symptoms, the ability of AFB to form spores makes it the worse of the two infections, as the spores are impervious to antibiotics and can continue infecting hives nearby for years to come. You can tell them apart by smell (EFB usually emits an extremely strong odour like off milk), or by inserting a matchstick into a cell with dead brood in it. If the goo inside the cell stays stuck to the matchstick and pulls out like a piece of string, you have AFB. It is also possible to send in a sample matchstick from hives you suspect may have been infected to a lab for testing. For instance in my state Biosecurity Queensland will do this test for free.
In terms of treatment there is not really that much you can do if you have AFB. In most parts of the world you are required by law to kill the bees and burn the beehive. This is because the dead hive will still be able to infect the bees from other colonies and spread the infection. It is possible to get away with just burning the frames if you treat the boxes with radiation or scorch them. For EFB you may be able to successfully treat the hive with antibiotics as they do not form resistant spores. Otherwise you could do a ‘shook swarm’ by shaking the bees on infected frames into a clean box with new frames. This has been shown to be successful and I have even heard of people doing this with AFB infected colonies as well.
The highly contagious nature of both these diseases is what is most damaging about them. You can even transfer infection between colonies through the tools and equipment you used. This is why you should be careful about buying second hand equipment but you can wash tools in ethanol or scorch them to be safe. Finally, be careful when feeding honey to weakened hives to supplement their diet as the infection is often spread through this medium.