Is plague the same disease everywhere?
Yes. Every plague case on this site, in every country, is caused by the same bacterium: Yersinia pestis. Two things do vary: where it attacks the body and which family branch of the bacterium is circulating in a region.
One germ
Plague isn't a group of similar illnesses. It's one species of bacterium, Yersinia pestis, which lives in small mammals and their fleas. A case only counts as confirmed plague when a lab finds this bacterium or its specific markers in a patient's sample.
That's why the Irkutsk death in Russia is not counted here: no lab has reported finding Y. pestis. Read why
Three forms, depending on where it attacks
Bubonic plague is in the lymph nodes, pneumonic plague is in the lungs, and septicemic plague is in the blood. They are the same infection, and one can turn into another: untreated bubonic plague can spread to the lungs or blood. The form matters because pneumonic plague can spread between people and is the deadliest without fast treatment.
WHO's 2019–2025 data shows how the forms break down where countries reported them. In Madagascar, 83.2% of suspected cases were bubonic and 15.5% were pneumonic. Pneumonic cases were far deadlier: 41.3% of them died, against 15.7% of bubonic cases.
Madagascar
1,482 suspected cases, 2019–2025
| Form | Cases | Deaths |
|---|---|---|
| Bubonic | 1,233 | 194 |
| Pneumonic | 230 | 95 |
| Other | 19 | 18 |
China
19 confirmed cases, 2019–2025
| Form | Cases | Deaths |
|---|---|---|
| Bubonic | 9 | 1 |
| Pneumonic | 6 | 3 |
| Septicemic | 3 | 2 |
| Other | 1 | 1 |
Mongolia
17 confirmed cases, 2019–2025
| Form | Cases | Deaths |
|---|---|---|
| Bubonic | 15 | 5 |
| Septicemic | 2 | 2 |
Uganda
2 suspected cases, 2019–2025
| Form | Cases | Deaths |
|---|---|---|
| Bubonic | 2 | 1 |
Source: WHO, Human plague, 2019 to 2025 (Table 11). Madagascar and Uganda reported forms for suspected cases; China and Mongolia for confirmed cases, so the countries aren't directly comparable. DR Congo did not report forms.
Family branches of the same bacterium
Like any living thing, Y. pestis slowly changes as it copies itself. Scientists read those changes in its DNA to build a family tree. That tree shows where the bacterium came from and how it traveled.
- It started in Asia. A 2010 genome study found that Y. pestis evolved in or near China and spread to the rest of the world in several waves. Each region's strains carry their own genetic markers. For example, U.S. strains belong to one branch and Madagascar's to another.
- Madagascar's plague arrived through a port in 1898. A 2025 study of 614 genomes found that all of Madagascar's strains belong to a single branch, called 1.ORI3, which points to one arrival at the port of Toamasina.
- The Black Death was the same bacterium. DNA from 14th-century victims in London showed that the medieval strain sits very close to the common ancestor of the strains that infect people today.
So a "strain" or "branch" is more like a family line than a different disease. It's how investigators can tell whether an outbreak came from local animals or was carried in from somewhere else.
Does the branch change treatment?
WHO says antibiotics are effective against plague, and that early diagnosis and treatment save lives. Drug resistance is rare but has happened: in 1995, a strain resistant to several antibiotics was found in a patient in Madagascar.
When "plague" means something else
In everyday speech, "plague" can mean any widespread disease, and it's also the name of video games and films. On this site, plague always means a human infection with Yersinia pestis reported by a health authority.