I think my two roosters have fowlpox. Here I am thinking I'm doing a good job taking care of my chickens, scooping the poop every morning, washing the waterers, doing the occasional check for lice and mites. They both have black spots on their combs.
I'm going to let you in on a secret: all of our birds get sick. Birds are going to pick up worms, lice, mites, viruses, bacteria, etc. Over here I'm constantly fighting worms and bumblefoot. If you saw some of my biddies feet, you'd think I was a terrible mom. But we do one at a time, and do it when they get bad. When they heal, the next one comes in for tuning up.
I had a GLW get black spots on her face and comb last fall, I swore it was fowl pox. It healed in a few days, nothing came of it.
I'll let you in on another secret: Every single one of us gets sick. Exactly two weeks ago I got quarantined by the CDC for catching a nasty protozoa. The viruses and bacteria that surround us are amazingly adept at making us sick.
So don't beat yourself up.