For centuries, Borna disease virus has plagued the livestock of Europe, leaving horses, sheep, cattle and other domesticated animals reeling from a bizarre and often deadly combination of neurological symptoms. Once stricken, usually by picking up the pathogen from an infected but symptomless shrew, animals would act aggressively, stagger about and smash their heads repeatedly into objects.

Slowly, the list of potential hosts began to grow. Cats, too, were vulnerable, researchers found, as well as dogs, foxes, primates and even birds. And when scientists began to experiment with the virus in the lab, they discovered that it could infect virtually any warm-blooded animal they tried.

The virus’ apparent ubiquity quickly sparked concern. Its hop into humans, some argued, seemed more a question of when than if.

Now, after years of fruitless searching for Borna in people, it’s clear that the virus indeed infects humans—and has likely been killing them for decades, reports Kai Kupferschmidt for Science magazine. In a study published this week in Lancet Infectious Diseases, researchers identified eight instances of lethal Borna disease in humans, roughly doubling the number of known infections in our species.

“Borna disease virus infection has to be considered a severe and potentially lethal human disease,” says study author Barbara Schmidt, a microbiologist at Regensburg University Hospital in Germany, in a statement.

But, on the whole, the average person’s risk of infection remains “pretty low,” study author Martin Beer, head of the Institute of Diagnostic Virology at the Friedrich Loeffler Institute in Germany, tells Tanya Lewis at Scientific American.

The findings come just five years after the first confirmed evidence of Borna disease virus entering the human population. In 2015, a strain of the pathogen killed at least four people after triggering severe inflammation, brought on by the immune system, in their brains. Three years later, another viral variant was discovered in the five other individuals, three of whom had recently received organ transplants, Lewis reports.

To better understand these patterns of infection, Beer and his colleagues searched for the genetic evidence of the virus in 56 samples of brain tissue collected in Germany between 1995 and 2018. All the patients had died from some kind of brain inflammation, which can result from autoimmune disease, cancer, infection and a variety of other conditions. Half the specimens had been logged without a known cause for the inflammation. In seven of these, the researchers discovered traces of Borna disease virus. An additional search at another German medical center turned up yet another case, bringing them to a total of eight patients, two of whom had been recipients of organ transplants.

Neither of the organ donors tested positive for the virus. And when the researchers sequenced the viral samples they’d extracted from the dead patients’ brains, they found the virus genomes bore relatively little resemblance to one another, suggesting each case of the disease made an independent jump from animal to person, rather than being passed from human to human.

Exactly how the transmissions occurred, however, is still up for debate, Beer tells Science magazine. Though bicolored white-toothed shrews (Crocidura leucodon) have previously been blamed for transmitting the disease to other animals, the sheer number of other species found to carry the virus leaves the human-infecting culprits mysterious. Five of the patients owned cats, at least two of which regularly gifted their humans with dead rodents and shrews.

Until more cases are identified, the method of transmission will probably remain mysterious, Norbert Nowotny, a virologist at the University of Vienna who wasn’t involved in the study, tells Science magazine.

So far, no known treatment for Borna disease exists, which seems to have a fairly high fatality rate across species (though a handful of human individuals have survived). But people shouldn’t panic: As Nowotny explains in a separate interview with Scientific American, the virus seems to have trouble traveling from person to person, and seems unlikely to cause an epidemic.