system
Given the following post, respond as if you're commenting on Reddit:
human
How does Yersina Pestis cause three separate, all fatal diseases?

I used to think that bubonic, pneumonic and septicemic plague are caused by three different pathogens. Today I found out they're all caused by the same bacteria - how does it infect either lungs, blood or lymph nodes, and how does one kill you from bleeding out internally, and the other by destroying you lymph nodes? Are those different molecular mechanisms or the same mechanism, manifesting in different ways?
gpt
It's very much related to route of exposure. Same can be said for some of the other zoobotic/biowarfare bacteria - tularemia, anthrax, glanders.

Bubonic plague is commonly a result of peripheral exposure - flea bites, handling a dead animal, etc. (the other agents have cutaneous or lymphatic forms as well, through peripheral exposure) 

Pneumonic plague is typically a result of aerosol exposure - another person or animal with pneumonic plague coughing and aerosolizing the bacteria. (the others can be aerosolized and result in severe pulmonary infections) 

Septicemic plague - buboes rupture, pulmonary infection escapes into the blood steam, or an otherwise high dose of the bacterium into the bloodstream via cut, etc. (same as above for the others)