She may have a very light case of fowl pox on her comb, but I don't believe that's what is make her sick. Put a little Blu-kote or iodine on the comb lesions and that will encourage them to dry up.
Your hen is sick from something else. You will need to help us figure it out.
How old is she? Is she laying? When was the last egg? What is her poop like? Does she have a watery discharge from her vent?
Do you have a compost pile where the chickens dig? Is there any moldy or rotten food disposed of there?
Have you checked the feed for spoilage and mold. It will smell awful if it's spoiled.
Look inside her mouth and throat. Open her beak wide. It won't hurt her. Look for any lesions or yellowish coating in the throat and tongue.