One bacterium, several illnesses. Which one a person gets depends on how the bacterium entered and how far it has travelled.
Bubonic plague — in a lymph node. The commonest form. After a flea bite the bacteria travel through the lymph vessels to the nearest lymph node and multiply there. The node swells and becomes intensely painful; that swelling is the bubo that gives the form its name. In the United States more than 80 % of cases are bubonic. It is the most survivable form, and it rarely passes from one person to another. Untreated, between 30 % and 60 % of patients die — and in some of them the infection first moves on, into the blood or into the lungs.
Septicaemic plague — in the blood. The bacteria multiply in the bloodstream. It can be the first form to appear, after a flea bite or after handling an infected animal, with no bubo at all. Or it can follow bubonic plague that was not treated: British guidance puts that at 10–20 % of untreated bubonic cases. Blood clotting fails; there is bleeding into the skin, and fingers, toes or the nose may turn black as the tissue dies. Untreated, it is fatal.
Pneumonic plague — in the lungs. The rarest form and the most dangerous. Primary pneumonic plague comes from breathing in infected droplets. Secondary pneumonic plague develops when untreated bubonic or septicaemic plague spreads to the lungs. It is the only form that passes from person to person, and it is the fastest: the World Health Organization calls it one of the most fulminating diseases known to humankind. Unless antibiotics are started within about a day of the first symptoms, it is almost always fatal.
Rare forms.
- Pharyngeal plague follows eating or drinking something contaminated: a sore throat with painful, swollen nodes in the neck.
- Plague meningitis is a complication in which the infection reaches the lining of the brain.
- Pestis minor is a mild illness — swollen nodes, fever, headache, exhaustion — that clears within about a week. It is described in places where plague is common.
The three main forms, side by side.
- Where the bacteria are: bubonic — a lymph node; septicaemic — the blood; pneumonic — the lungs.
- The usual route: bubonic — a flea bite; septicaemic — a flea bite, handling an infected animal, or spread from a bubo; pneumonic — breathed-in droplets, or spread from another form.
- Passing from person to person: bubonic — rarely; septicaemic — no; pneumonic — yes, by cough, at close range.
- Without treatment: bubonic — 30–60 % die; septicaemic — fatal; pneumonic — almost always fatal.
One form can turn into another. That is why the form a person starts with is not the form to plan for. Someone with a fever and a bubo who waits may, a few days later, have plague in the blood or in the lungs — and from then on may also be infectious to the people looking after them.
Source. WHO, Plague fact sheet, https://www.who.int/news-room/fact-sheets/detail/plague ; United States Centers for Disease Control and Prevention, Signs and symptoms of plague, https://www.cdc.gov/plague/signs-symptoms/index.html , and Plague: maps and statistics, https://www.cdc.gov/plague/maps-statistics/index.html ; UK Health Security Agency, Plague: epidemiology, outbreaks and guidance, https://www.gov.uk/guidance/plague-epidemiology-outbreaks-and-guidance ; MSD Manual, consumer version, Plague and other Yersinia infections, https://www.msdmanuals.com/home/infections/bacterial-infections-gram-negative-bacteria/plague-and-other-yersinia-infections
What this is not. Reading, not medical advice and not a way to diagnose yourself or anybody else.