The more it goes, the more I think this "plague doctor" is nothing but a 15th century scammer who began to believe his own lies.
While it's evident this guys is anomalous in his own way (coat and mask symbiosing with his body, near-immortality), I think his zombification process is mostly an alternate use of SCP-008 (zombie plague).
According to the new report, he shows suspecious signs of denial over his own "cure", so I think he's just is lying to himself.
Also, interesting videos to try to decypher the mystery :
https://www.youtube.com/watch?v=jwia6wM1QXs
https://www.youtube.com/watch?v=1MU7s8zTDtk
https://www.youtube.com/watch?v=ixuHTC8-nMo