Nearly a quarter of London’s population died when the Great Plague struck the city in 1665 — and it turns out it never left.

On Thursday, archaeologists isolated the origin of the gruesome epidemicYersinia pestis — which manifested in painfully swollen lymph nodes and blood gushing from the body’s orifices. This is both good and bad news: While it’s useful to know what 17th-century Londoners were up against, it’s more than a little scary to realize that this microbe has not been lost to history.

In fact, more than a few cases have popped up in the United States over the past couple of years. In 2015 alone, the CDC reported 15 cases, four of which were fatal. And just as in London’s Great Plague, the disease’s spread ultimately came down to the microbe’s reviled carrier: rats.

London's Great Plague killed an estimated 100,000 people between 1664-1665.
London's Great Plague killed an estimated 100,000 people between 1664-1665.

The plague spreads when infected rat fleas tiny bugs that live in the fur of infected rats — leap from their rodent hosts to humans and transmit Yersinia pestis into the bloodstream. Squirrels and chipmunks, together with infected domestic cats, can carry the fleas, too.

Fortunately, these days, infection with plague can be treated with antibiotics if diagnosed early enough. This isn’t always easy: Because the plague’s initial symptoms — that is, before the swelling and bleeding — are flu-like, most people don’t realize that they, in fact, have the plague.

Yersinia pestis is transmitted via the rat flea, which (still) lives on plague-infected rodents.
Yersinia pestis is transmitted via the rat flea, which (still) lives on plague-infected rodents.

While there’s no reason to fear a sudden epidemic anytime soon — the plague’s spread largely depends on the interactions between fleas, rodents, and humans, and we haven’t been especially close, lately — we’re still at risk. In the U.S., the western states are most likely to see spurts of the plague. Specifically, New Mexico, Arizona, and Colorado are at risk (though in 2015 cases were reported in Oregon and California).

It’s been argued that, in the U.S., racist public health schemes are responsible for the disease’s origin and spread: In 2015, Slate outlined the “bungled” efforts of early officials to kill and contain rats in California’s Chinatown, thought to be the early epicenter of the disease:

We missed our window to eradicate plague in the United States. By the time we realized rodents were infected and spreading the organism, the genie was out of the bottle, and the political will necessary to marshal an elimination campaign never materialized.

So far this year, three cases of the human plague have been identified in New Mexico alone; thankfully, no fatalities have been reported thus far.

Yasmin is a writer and former biologist living in New York. A Toronto girl at heart, her writing also appears in The Last Magazine and SciArt in America. You might recognize her as a past host of Scientific American's YouTube series.