There are three routes that matter, and one of them accounts for most cases.
The flea. Most human plague starts with the bite of a flea that has fed on an infected rodent. Inside the flea the bacteria multiply and block its gut. A blocked flea cannot feed properly, so it bites again and again, and passes bacteria into each bite. A flea bite leads to the bubonic form of the disease.
Why dead rodents matter. Wild rodents often carry the bacterium without falling ill. Rats that live beside people do fall ill, and die — and when a rat dies, its fleas leave the cooling body and look for the nearest warm one. A sudden die-off of rats is the classic early warning that plague is active in a place. The World Health Organization calls it a "rat fall": more than one dead rat in a house, or dead rats in more than one house, with no poison to explain it. The risk to people is highest during such a die-off and just after it.
Handling an infected animal. The bacteria are in the blood and tissues of an infected animal. Skinning or butchering one with bare hands — a hunted rabbit, a marmot, a ground squirrel — lets them in through small breaks in the skin. Eating infected meat that has not been cooked through is a rarer route.
Breath. When the infection reaches a person's lungs, that person coughs out droplets carrying the bacterium, and someone close by who breathes them in can develop plague in the lungs directly, with no flea involved. This is the only way plague passes from one person to another in the ordinary sense, and it needs close contact: guidelines draw the line at about 2 metres. Animals can do the same. Among domestic animals it is most often cats, which catch plague by eating infected rodents and can then infect an owner or a veterinarian.
What rarely or never spreads it.
- A person with bubonic plague and no cough rarely infects anyone. The risk is direct contact with pus from the swollen lymph node.
- Surfaces. The bacterium does not persist on dry, inert surfaces; the World Health Organization states that disinfecting the houses of people with plague is useless for that reason.
One exception to remember. The body of a person who has died of plague can infect the people who wash it or prepare it for burial. That is why relatives are asked not to do so.
Who is more exposed.
- People who hunt, trap, skin or butcher wild animals.
- People who camp, hike or work outdoors where plague exists in animals.
- Households with rats or a heavy flea infestation.
- Owners of cats and dogs that roam and hunt in such areas.
- Veterinary, laboratory and health-care staff.
Source. WHO, Manual for plague surveillance, diagnosis, prevention and control (2024), sections 2.2–2.6, https://www.who.int/publications/i/item/9789240090422 ; United States Centers for Disease Control and Prevention, How plague spreads, https://www.cdc.gov/plague/causes/index.html ; UK Health Security Agency, Plague: epidemiology, outbreaks and guidance, https://www.gov.uk/guidance/plague-epidemiology-outbreaks-and-guidance
What this is not. Reading, not medical advice. Where your health authority gives instructions, follow them.