Throughout duration of illness (pneumonic plague is the most important infectious form).
Transmitted from rats by flea bites, by handling infected animal tissues, or airborne by aerosol from animals or humans with pneumonic plague.
There are natural foci of plague infection of rodents in many parts of the world. Wild rodent plague is present in central, eastern and southern Africa, South America, the western part of North America and in large areas of Asia. In some areas, contact between wild and domestic rats is common, resulting in sporadic cases of human plague and occasional outbreaks.
In Southeast Asia, Myanmar and Vietnam report the highest number of cases.