On May 31, 2017, 58-year-old Tamela Wilson checked into Barnes-Jewish Hospital, in St. Louis, with a fever, fatigue, and a strange red rash. She’d been undergoing chemotherapy to treat a relapsing lymphoma, but this exhaustion wasn’t just the cancer or the drugs. She told the doctors she worked at nearby Meramec State Park, tending to its miles of trails through forested river bluffs. And that while there, a week before her symptoms started, she found two ticks burrowed into her body.

Wilson’s doctors tested her blood for Missouri’s common tick diseases. On day three, when the results came back negative, they sent a vial to Fort Collins, Colorado, to the Centers for Disease Control and Prevention’s Division of Vector-Borne Diseases, for a more extensive analysis. On day 10, they got their answer: Bourbon virus. Thirteen days later she was dead.

At the time, Wilson was one of only a handful of people in the world to ever be diagnosed with a Bourbon virus infection, and the second to succumb to the mysterious microbe since it was discovered in 2014. Two years later, the strain of virus that once coursed through Wilson’s veins lives on in freezers and petri dishes a few buildings over from where she died, as researchers race to find possible treatments for the next time.

Scientists know almost nothing about how Bourbon virus behaves or how it got here or where it will show up next. But they have learned enough to know they haven’t seen the last of it.

What they fear most is that the virus could be silently spreading through human populations, getting noticed only when it causes severe symptoms in an unlucky few. That’s what’s started to happen with another new tick-borne virus in Missouri. If Bourbon virus were to establish itself there in humans too, the chances go up that more cases, like Wilson’s, will turn deadly. And until someone conducts the necessary studies, there’s no telling how far the virus has spread or how many people might potentially be in danger.

Under a microscope, Bourbon virus is a shape-shifter, sometimes long and filamentous, sometimes a sphere studded with spiky proteins, encapsulating a string of segmented genetic code. It’s the only human pathogen of the genus Thogotovirus to make it to the New World. Its nearest evolutionary relatives are viruses found in the bodies of sheep-sucking ticks in Kyrgyzstan and camel-chewing ticks in India, both of which attack neurons and cause brain inflammation when transmitted to humans. Bourbon virus appears to have developed a taste for other types of human cells during its travels; in the few documented cases that have appeared in the US, patients have experienced massive declines in their white blood cell populations.

Like its closest cousins, Bourbon virus seems to spend at least some of its time in ticks. The patient the virus was first isolated from—a 68-year-old man named John Seested in Bourbon County, Kansas—had a history of tick bites. The summer after its discovery there, CDC researchers found the virus in the bodies of several ticks collected elsewhere in Bourbon County. The species they found carrying the virus was the Lone Star tick, whose bite is more notorious for making people allergic to red meat. It’s also been shown to replicate inside tick cell lines in the lab.

But the CDC has yet to formally declare Bourbon virus a tick-borne disease. To definitively link Bourbon virus with its suspected vector, the agency needs more data—specifically on how well Lone Star ticks acquire, maintain, and transmit the pathogen in a lab. Aaron Brault, a microbiologist with CDC’s Division of Vector-Borne Diseases, says those studies are currently in progress. He says it’s now a “strong probability rather than simply a possibility” that the virus is transmitted to humans through ticks.