Aah, well. Bubonic plague is endemic in my home state of New Mexico. With antibiotics, it is successfully treated, and the combination of modern living conditions, among with our sparse population make it a non-issue as an epidemic.
Much of its character as an infectious disease is related to the condition of public hygiene and the crowding of the poor. It is in the slums and waterfront districts that plague can take hold, and the Medici family focus on the care of paying customers had left these poorer quarters vulnerable.