The case adds yet another terrifying layer to the Ebola outbreak: A Scottish nurse who recovered from Ebola 10 months ago has been rehospitalized and is now critically ill, the Royal Free Hospital in London reported Wednesday.

Scientists have long known that the Ebola virus can persist for months in certain tissues of the body that are relatively protected from the immune system, including the eyes and the testes.

A report published Wednesday in The New England Journal of Medicine, for instance, found traces of Ebola in some men’s semen up to nine months after they fell ill. Another report in the same journal presented additional evidence that an Ebola survivor in Liberia had transmitted the disease to a sexual partner roughly six months after having developed symptoms.

Yet the case of the nurse, Pauline Cafferkey, points to how much is still unknown about the virus and its long-term effects.