The facultative anaerobic, Gram-negative rods are transmitted in flea regurgitant during bites.
Septicemic plague occurs when the bloodstream is invaded.
Because an untreated bubonic plague epidemic almost inevitably transforms into septicemic and then pneumonic plague, all plague outbreaks are considered serious public health risks. The common image of the Dance of Death grew out of the nightmare of plague in the Middle Ages. The bacillus apparently incorporated a resistance plasmid from another species of bacteria, further fueling fears of plague as a disease that could reemerge catastrophically in the future, especially as antibiotic resistance increases worldwide. Preventing plague, as with preventing endemic rabies, requires monitoring the vectors and maintaining sanitary conditions rather than using vaccination. Generally available plague vaccines are effective against only the bubonic form of plague and are somewhat unreliable.