Yesterday morning, I published a story about the silent spread of resistance against the antibiotic of last resort, colistin—a major step toward the emergence of a superbug resistant to all antibiotics. While reporting this story, I interviewed Alex Kallen, an epidemiologist at the CDC, and I asked if anyone had found such a superbug yet. “Funny you should ask,” he said.

Funny—by which we all mean scary—because yesterday afternoon, the CDC also released a report about a Nevada woman who died after an infection resistant to 26 antibiotics, which is to say all available antibiotics in the U.S. The woman, who was in her 70s, had been previously hospitalized in India after fracturing her leg, eventually which led to an infection in her hip. There was nothing to treat her infection—not colistin, not other last-line antibiotics. Scientists later tested the bacteria that killed her, and found it was somewhat susceptible to fosfomycin, but that antibiotic is not approved in the U.S. to treat her type of infection.

The woman was isolated so that her superbug would not infect other patients in the hospital. And subsequent samples from other patients near her in the hospital have not turned it up. If this superbug is somehow gone from the hospital and gone from the U.S., that would be great news. But even if so, other pan-resistant superbugs are likely to emerge.

Here’s why: The most worrisome kind of colistin resistance is caused by a single gene called mcr-1. The bacteria that killed this woman did not have mcr-1; it’s still unclear how they became resistant. Other cases of colistin resistance have emerged before though. What makes mcr-1 special is that sits on a loop of free-floating DNA called a plasmid, which bacteria of different species can pass back and forth. And there are many plasmids out there with genes that confer resistance to this or that class of antibiotics.

Where might bacteria go to hang out and swap plasmids? Well your gut is a big bag of bacteria. One day, you might pick up some bacteria with a plasmid carrying resistance to colistin. Years later, you might pick up some bacteria with a plasmid carrying resistance to carbapenems. And so. They start swapping plasmids. All this time you are healthy, and these bacteria just lurk in your gut, not causing much trouble. Then you get sick, your immune system is down, and you take antibiotics for an infection. The antibiotics kill everything but the resistant bacteria, which have by now collected all the resistance genes and no competition. That’s how you get a pan-resistant infection.

The danger isn’t just that a single pan-resistant bacteria emerges and terrorizes the world. It’s that pan-resistant bacteria can keep emerging independently. The nightmare might go away, only to come back somewhere else.