The bacterium
Plague is caused by Yersinia pestis, a small, non-motile, Gram-negative bacterium named after Alexandre Yersin, who isolated it in Hong Kong in 1894. It evolved from the gut bacterium Y. pseudotuberculosis several thousand years ago. Outside a host it is fragile: sunlight, heat and drying kill it quickly.
Rodents and fleas
Plague lives in natural foci, places where it circulates among wild rodents such as marmots, ground squirrels, gerbils, prairie dogs and rats, carried between them by fleas. Most of the time it causes no human illness at all. WHO notes that as an animal disease it is found on all continents except Oceania. Russia alone has 11 officially mapped foci covering 222,377 km2.
How humans get it
- the bite of an infected flea, the most common route
- handling or skinning infected animals, or eating their undercooked meat (marmots in Mongolia and the Altai)
- contact with infected body fluids or laboratory material
- inhaling droplets from a person or animal (often a cat) with pneumonic plague, the only route between people
Three forms
Bubonic plague (lymph nodes, 80 to 95 percent of cases), septicemic plague (bloodstream) and pneumonic plague (lungs). Untreated, bubonic plague kills 30 to 60 percent of patients and the other two are always fatal. Treated early with antibiotics, most patients survive. See symptoms and treatment.
Plague today
Between 2019 and 2025, ten countries reported 3,860 suspected human cases to WHO, with 423 deaths. DR Congo and Madagascar account for almost all confirmed cases; Kenya, Mongolia, China and Uganda reported the rest. The US averages about seven cases a year but sent no data to WHO for those years. WHO stresses that "absence of reporting does not imply absence of cases".
In October 2026 a suspected pneumonic plague death at an anti-plague institute in Irkutsk, Russia, put the disease back in the headlines. Authorities have not confirmed plague. See the Irkutsk briefing.
Common myths
"Plague is a medieval disease."
It remains endemic in several countries. WHO warns that quiet periods "may lead to the erroneous belief that plague has been eradicated until a new outbreak occurs".
"Plague cannot be treated."
Common antibiotics cure it. With treatment, bubonic plague fatality falls from about 66 percent to 13 percent (CDC).
"It spreads through the air like measles."
No. Only pneumonic plague spreads between people, through large droplets at close range. There is no evidence of airborne spread over distances.
"Kill the rats first."
WHO says flea control must come before rodent control, because killing rodents first sends their fleas looking for new hosts, including people.
Reference information, not medical advice. With symptoms after a possible exposure, call your local emergency number.