Fowl Pox is a really interesting virus. It's categorized as a large DNA virus and can have integrated Reticuloendotheliosis Virus in it. That addition to Fowl Pox can lead to lymphoma. It also differs significantly in it's makeup between field and lab strains. Field strains have cell adaptations that allow it to persist much longer in the environment. It's also very slow to spread (which means vaccination is still useful even when some of the flock is symptomatic)
There are three types of Fowl Pox, cutaneous, diphtheritic, and/or systemic. The first one results in the classic skin lesions (dry fowl pox) and has low mortality so unless the bird is very young, old or immunocompromised, full recovery is likely. They then have passive immunity but this can decline over time, opening the bird up to re-infection. Interestingly, just like with human chicken pox, mothers can pass immunity to their offspring.
The other two versions are much more dangerous. Diphtheritic (wet fowl pox or fowl diphtheria) results in lesions which develop on the mucous membranes of the mouth, oesophagus, pharynx, larynx, and trachea. Caseous patches can form in the larynx and trachea, they look like the plaques caused in oral thrush or trich and prevent the chicken from breathing. They adhere very strongly to mucous membranes. The third type, systemic, results from very virulent forms of the virus. It causes lesions on the internal organs. Both types have high mortality rates. It's also possible for an individual chicken to have all three forms at once.
Mosquitoes are the the main mechanical vector of the virus but other biting insects can also spread it. It can also be spread via pox scabs, dander and dust through skin to skin contact, via abrasions and cuts. It can also be transmitted by breathing in the virus present in the scabs that fall off the infected birds. When it is transmitted in this way, the result is almost always diphtheritic or systemic infection.
That's just a few highlights. It's a very well studied virus with lots of information out there as it has a worldwide distribution and can result in considerable economic loss in commercial settings.