This is something you do not need to cull for.
Fowl pox is pretty common and once they get over this, they can never get it again.
Yes with wet pox there is a chance it could be fatal.
But, There are a few people on here who have nursed their chickens back to health that had wet pox.
Separate the chicken from the others, so you prevent spreading it.
If the other chickens do get it, it doesn't necessarily mean they will get the wet form.
Wet pox can make it painful for them to eat, drink, and breath so you may end up having to force feed your chicken so she doesn't starve to death.
When my chicken had fowl pox, i fed her lots of scrambled eggs and yogurt.
You can even put vitamins and electrolytes in the water to boost the immune system and hopefully prevent a secondary infection.
I didn't do this, but i heard you can put iodine on the plaques, reference:
https://www.backyardchickens.com/t/98843/can-iodine-be-used-on-wet-pox-mouth-ulcers-update.
I wish you and your chickens well!