If you notice your chickens developing white spots on their skin, scabby sores on their combs, white ulcers in their mouth or trachea, and their laying stops then you should grow concerned that your chickens are developing Fowl Pox.
There are treatment options for Fowl Pox. You can feed them soft food and give them a warm and dry place to try and recoup. With adequate care, there is a great chance that your birds can survive this illness.
If you would like to remove the odds of your birds even contracting this disease there is a vaccine available.