Once a major outbreak dies down, getting rid of a disease entirely is a game of whack-a-mole—and in Ebola’s case, the mole is a deadly, gruesome virus scientists are just beginning to understand. Last Friday, London’s Royal Free Hospital announced that it was treating Pauline Cafferkey, a Scottish nurse who had served in Sierra Leone during this year’s West African outbreak, for what they termed “an unusual late complication” of Ebola. Somehow, the Ebola virus was once again raging through her system, nine months after her initial infection and recovery.

The case is dismaying, but it’s no freak occurrence. Even though the worst of the Ebola outbreak is over, the virus keeps reappearing—in survivors, new patients, and the press. In the past 24 hours, Ebola has struck two people in Guinea, and a paper out this week in the New England Journal of Medicine announced that Ebola patients still housed traces of the virus’s RNA up to nine months after they first showed symptoms. And even if they’re not wracked by the disease anymore, Ebola survivors suffer a whole range of maladies that come from the lingering virus: back pain, hearing loss, meningitis, seizures. (Though, thankfully, the survivors probably aren’t infectious.)

The WHO counts 42 days without new cases as the cut-off for a region to be Ebola-free (Guinea was weeks away), but they may need to rethink that length of time, or the very idea that a region can be Ebola-free, says Dan Kelly, an Ebola researcher at UC San Francisco.  To stretch the mole analogy, the squiggly virus collects in certain hidey-holes the immune system doesn’t patrol as well—eyes, brains, testes, and even semen—where it can then lurk for months before replicating and causing problems for its host. (In another study this week in NEJM, scientists found that Ebola can be transmitted through sex, which presents a whole ‘nother set of risks.) Scientists still don’t know how long Ebola stays infectious in the body.

So, even if you fight off the infection once, Ebola can still live and grow inside of you. That’s scary, Kelly says. “The immune system didn’t do a great job of curing itself—how do we know giving Pauline Cafferkey supportive care will cure her of Ebola? We don’t.” (Supportive care, as opposed to, say, therapeutics or steroids to give the body a boost.) Of course, this relapse has a dark silver lining: When people used to get Ebola, they’d die. Now, with high-profile survivors like Cafferkey and other health workers, scientists have a better view of the disease’s long tail—which, in the end, might lead to cures.