A Scottish nurse who recovered from Ebola nine months ago has been re-hospitalized and is now critically ill, London's Royal Free Hospital announced on Wednesday.

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

The doctors are finding it hard to understand why nurse Pauline Cafferkey's current illness happened so many months after her initial infection in Sierra Leone, where she had gone as a volunteer, and why similar cases have not been well documented in the 3 West African nations that are now home to thousands of survivors.

In the past 24 hours, Ebola virus has infected two people in Guinea, and a report published this week in the New England Journal of Medicine announced that Ebola patients still had traces of the virus?s RNA up to 9 months after they first showed symptoms.

Even if Ebola survivors are not wracked by the disease anymore, they suffer a whole range of illnesses that come from the lingering virus: meningitis, seizures, back pain and hearing loss. However, the survivors probably are not infectious.

Dan Kelly, an Ebola researcher at UC San Francisco, said the World Health Organization (WHO) counts 42 days without new cases as the cut-off for a region to be Ebola-free, but they might have to reconsider that length of time, or the very idea that a region can be Ebola-free.

To stretch the mole analogy, the squiggly virus collects in certain hidey-holes the immune system does not patrol ? testes, brain, eyes, and even semen - where it can then lurk for months before replicating and causing problems for its host.

It has been found that Ebola virus can be transmitted through sex, which presents a whole range of health risks. Scientists still do not know how long Ebola stays infectious in the body, according to another study published in the journal NEJM.