im pretty sure the Iva pox is the black plague it hadnt been seen in 1000 of years and no cure was made and the symptoms are quite the same to one another
(I made an account to comment on this ^^') The bubonic plague has a vaccine made for it, antibiotics can easily treat it, there are some recent cases (although we haven't seen epidemics or pandemics of it), buboes are much larger than a simple rash and, if I recall correctly, pox of any kind is caused by viruses. In short - I doubt Yersinia Pestis is involved in avia pox.