It is carried by mosquitoes or other chickens picking infected scabs. The Dry form is more just an irritation, unless it forms around the eyes badly. If it forms around the eye and causes a lot of swelling and pressure on the eye, then a good antibiotic may be needed. My little Silkie, Sophie got it badly around one eye and it swelled very badly, I had to put her on Baytril and it went down but she now has a blue cloud in that eye.
In the wet form of Pox they get more respiratory type symptoms, bad scabs about the eyes and canker type puss pockets in the throat and nose, that must be removed or it can block air movement and food. If it goes wet then antibiotics are necessary.