F/B is extremely contagious. There is no cure. The first warning sign is SMELL. (Remember, Look, Listen and Smell). Whenever you get into the brood chamber, inspect the sealed brood. If the brood caps are broken and the larvae is rotting in the cell, it is probably F/B.
THE FINAL TEST IS TO TAKE A TWIG, PUT INTO THE DECAYING LARVAE AND PULL OUT. IF IT PULLS OUT LIKE A STRING, YOU HAVE AMERICAN FOULBROOD.
I repeat, THERE IS NO CURE! If you discover an infected hive, be aware that everything you have used is now contaminated. Especially your hive tool, hands, etc. I keep alcohol with me for such an occasion.
You must destroy the hive. Do this in the evening, after flight has stopped. Use a pump up type sprayer filled with soapy water and pull every frame and spray the flightless bees. They will quickly drown. Put everything into double, heavy duty garbage bags. In the fall you will burn everything in a BIG bonfire. Textbooks will say you can salvage the boxes (only) by scorching the inside with a plumber's torch, but I would not trust it.
Foulbrood is spread by robbing. As the hive weakens (because there is no brood hatching), it will be robbed out by other hives, infecting them. This is why it is SO CRITICAL TO CATCH A DISEASED HIVE EARLY.
ALSO BE AWARE THAT IT IS THE STRONGEST HIVES THAT OFTEN CATCH FOULBROOD. This is because they are strong enough to overwhelm a weakened (diseased) hive.
There is a medication, terrimyicin (sp?), that PREVENTS F/B. Many beekeepers use it in the spring and fall, in the form of patties. I do not. However, if I discover F/B, I do treat all my hives in the yard, since I am convinced there is a feral hive in the area that may infect the rest of my yard. I will post the recipe for patties in a comment below.
One other thing. So now you have these bags of rotting hives sitting in your garage waiting for the burn ban to be over. In September, you have forgotten all about it and left your garage door open, only to discover hundreds of bees flying into the garage and lapping up the honey that is seeping out of the garbage bags. NOT GOOD!