I'm currently working on a bubonic plague study.

All three variants are the same bacteria: y. pestis. In the most common case, bubonic plague, the bacteria reproduce and fill lymph nodes. (buboes)

This is the majority of all plague cases through history. When this form of plague, through chance, invades the lungs, only then does it get classified as "pneumonic". The more cases of plague there are, the more chances there are for this to happen. Outbreaks of plague are watched intently because of this possibility, as it can then spread from person to person, with no fleas in between. It's also much more deadly.

In two outbreaks, the Justinian plague and the Great Death (1340s) it's thought that a specific strain of bacilli, originating in the Asian steppes, which is more likely to shift to the pneumonic form, was responsible. There's a reservoir of this strain there to this day.

This is why contemporary accounts say "talking together, with breath, all will be stricken"- the very large number of infections at the time suggests that more will become pneumonic, and that those will spread faster. This likely accounts for the mass amount of deaths so quickly.

The Justinian plague in particular, given the estimated death rate, must have been this strain. In 1340s, it's debatable- but there's good reason to think the "Marmot Plague" variation was responsible.

More recent outbreaks have had only very rare cases of this pneumonic infection. Usually they are stopped before they reach the point of probability for it to occur.

The third form of plague, septicemic, is even more rare. It kills within a few hours, and is a result of the bacteria spilling into the bloodstream. In these cases it's always fatal, and there's some evidence it can be spread both person-to-person and by vector insects like the human flea, domestic pet fleas. It's so rarely seen in modern medicine that we don't have good data on it, only historical accounts.

Keep in mind that yersinia pestis is the only bacilli. Bubonic, pneumonic, and septicemic plague are caused by the same bacteria, behaving in different ways. They are not separate diseases.

-The Black Death, a history from beginning to end

-The Great Mortality, Kelley

-The Black Death, Hecker

-Rabid, Wasik/Murphy

-Medieval Medicine, Walsh

edited for grammatical clarity and spelling.