Well I mean it just popped up out of no where.
Have you brought home any new chickens? Been around other people's chickens? Shows, swaps auctions you should shower change clothes, even feed stores you don't want to ware your same shoes into the coop. I hope Oxine works for you, great stuff to have around only need 1/8 tea per gallon spray your shoes with, and clean coop with.
https://sites.google.com/a/poultrypedia.com/poultrypedia/chicken-fungal-infections
https://www.backyardchickens.com/t/...-poultry-often-mistaken-for-crd-mycoplasmosis