Plague is an infection caused by a bacterium, Yersinia pestis. It is first of all a disease of small wild animals — rodents above all — and of the fleas that live on them. A person is an accidental host. The bacterium reaches people when a flea that has fed on an infected animal bites them, when they handle an infected animal, or when they breathe in droplets coughed out by a person or an animal whose lungs are infected.
It is not a disease of the past. The same bacterium caused three pandemics. The first began in the year 541. The second included the Black Death of 1346–1353, which killed about a third of the population of Europe. The third spread around the world by steamship and railway at the end of the nineteenth century. The bacterium was identified in 1894 by Alexandre Yersin, and in 1898 Paul-Louis Simond showed that the rat flea carries it. It has never been eradicated, because it does not depend on people: it lives on in wild rodents.
Where it lives now. As a disease of animals, plague exists on every continent except Oceania. An area where the bacterium, a suitable rodent and a suitable flea are all present is called a natural focus. Since 2000 more than 95 % of human cases have been reported from Africa. Cases are also reported regularly from the Americas, and Central Asia has the largest number of natural foci, although few people live near them. Between 2000 and 2018, 26,237 human cases were reported to the World Health Organization from 21 countries. A focus can stay silent for decades and then produce cases again, which is why "there has been no plague here for years" is not the same as "there is none".
It is curable. This is the most important sentence in the course. Plague is a bacterial infection, and common antibiotics cure it — if treatment begins early. Untreated, the commonest form kills between 30 % and 60 % of the people who catch it, and the form that reaches the lungs is almost always fatal. Treated in time, most people recover. Nearly everything in the lessons that follow is about that one word, early: knowing the routes, knowing the incubation periods, knowing the first signs, and knowing what to do in the first hour.
What makes it dangerous is speed. The bacterium itself is fragile. Sunlight, heat and drying destroy it, ordinary disinfectants kill it within minutes, and it does not persist on dry surfaces. What it does is move very quickly inside a body.
Source. WHO, Plague fact sheet, https://www.who.int/news-room/fact-sheets/detail/plague ; WHO, Manual for plague surveillance, diagnosis, prevention and control (2024), https://www.who.int/publications/i/item/9789240090422 ; Barbieri R. et al., "Yersinia pestis: the natural history of plague", Clinical Microbiology Reviews 34(1), https://pmc.ncbi.nlm.nih.gov/articles/PMC7920731/
What this is not. Reading, not medical advice and not a diagnosis. If you are ill, a doctor decides. Where your health authority says something different from this page, the authority governs.