A flea, having ingested plague-infected blood from its host, can live for as much as a month away from that host before he needs to find another warm body to live on. When a blood-engorged flea attempts to draw blood from another victim, it invariably injects into that victim some of the blood already within it. If the injected blood contains the bacterium yersinia pestis , the result is Bubonic Plague. Fleas were, alas, such a part of everyday life that no one noticed them much. In this invisible manner the plague spread from rat to human and to cat and dog, as well.
Pneumonic plague is airborne.
The pneumonic form was much more virulent and spread much more quickly and just as invisibly. Plague is occasionally transmitted by direct contact with a carrier through open sores or cuts, directly into the bloodstream. This could result in any form of the plague except pneumonic, although it is likely that such incidents most often resulted in the septicemic variety. The septicemic and enteric forms of the plague killed most quickly of all, and probably accounted for the stories of individuals going to bed apparently healthy and never waking up. Some of the links in this feature take you to sites within a removable navigational frame.