Scottish nurse Pauline Cafferkey was one the 28,000 people infected with Ebola in last year's outbreak of the virus that was responsible for killing more than 11,000 people. The nurse contracted the virus while volunteering at a hospital in Sierra Leone last December and was treated soon after at the Royal Free hospital in London.

After three weeks of treatment, Cafferkey was declared Ebola-free and was discharged. But now, 10 months after being treated for the virus, Cafferkey has been admitted to a hospital in Scotland where she is "critically ill," raising many questions about the lingering effects of the virus. Here's what we know so far.

Researchers from the World Health Organization have discovered that many Ebola patients suffer long-term complications after undergoing successful treatment. In a study conducted in August among Ebola survivors in West Africa, half reported suffering from chronic problems, most commonly joint pain, headaches and eye inflammation that can cause blindness.

These after-effects are caused by the intense immune response elicited by the body after initial infection. The immune system can suffer trauma when fighting off Ebola, which makes survivors more susceptible to autoimmune diseases among other health problems afterward. Additionally, many survivors have also reported symptoms of tiredness that are similar to chronic fatigue syndrome.

Dr. Ian Cozier, an American physician who contracted the virus while in Sierra Leone, experienced some of the most severe after-effects of Ebola firsthand. Two months into recovery, Cozier began to experience blurred vision and a burning sensation and extreme pressure in his eyes and was admitted back to the hospital. Cozier had also observed that one of his normally blue eyes had turned green.

Upon testing, doctors discovered Cozier's eye was teeming with live Ebola virus, causing one of his eyes to appear green. Before observing Cozier's case, little was previously known by doctors and researchers about the potential for the virus to live on inside the eye. Cozier, however, was not at large risk of infecting those in contact with him because the virus had not infected Cozier's tears.

The virus has also been discovered to live on in semen and vaginal fluids, including ocular fluid and spinal fluid, months after recovery, which present a much greater chance of transmission. In a recent study published by the New England Journal of Medicine, 93 men in Sierra Leone who had recovered from Ebola were tested for the virus in their semen.

All of the semen samples tested positive for Ebola after three months of recovery, and one man continued to test positive after nine months. The World Health Organization recommends all male survivors be tested three months after experience virus symptoms and monthly from then on to make sure they have no risk of spreading the virus through their semen.

At least 20 cases of Ebola are believed to have been contracted through sexual transmission, and experts believe it may have been the reason for the re-emergence of the virus in Liberia in June after being declared Ebola-free. In an effort to educate survivors and prevent future outbreaks, Sierra Leone officials have established a testing program to monitor the ongoing health of survivors.

Although transmission of Ebola through bodily fluids is believed to be rare, the discovery of a delayed transmission after a supposed full recovery highlights the need for more research on the virus. With 17,000 Ebola survivors, the possibility of those individuals infecting others could persist if proper precautions are not taken.

The U.S. National Institutes of Health will continue to monitor the health of Ebola survivors and their close contacts in hopes of learning as much as possible about the long-term effects. Cafferkey will also be closely monitored as it is currently unclear whether she has a repeat infection of Ebola — which would be the first such instance ever documented — or is experiencing severe after-effects.

Nevertheless, the readmission of Cafferkey is concerning to scientists and the public alike as it provides us with a reminder of just how little is known about the long-term effects of Ebola.