[tet68]

As I recall the pneumonic plague is not spread by fleas.

[Valpal1]

The fleas start it by transmitting the bacteria to humans and and then it spreads from human to human via droplets if/when the infection spreads to the lungs.

[SunkenCiv]

Can be, it's just the really contagious airborne version of the same Plague, found in people who have contracted Plague any old which way. I learned that as a consequence of watching Hawaii 5-0 (the original series).