Ebola is back, and in a very frightening way.

Tragic and terrifying news. 

A British nurse who was successfully treated for Ebola in January is now critically ill after a resurgence of the virus, sparking fears for the thousands of survivors of the potentially deadly infectious disease.

Pauline Cafferkey contracted the virus while volunteering in Sierra Leone and was readmitted to the infectious diseases unit of London’s Royal Free hospital earlier this month, nine months after she recovered from the illness.

“We are sad to announce that Pauline Cafferkey’s condition has deteriorated and she is now critically ill,” the hospital said in a statement, the ABC reports.

“Pauline is being treated for Ebola in the high-level isolation unit at the Royal Free Hospital.”

The case – believed to be the second known case of “reactivated” Ebola – has shocked medical staff and raised concerns about the implications for other survivors, especially those in the hardest hit African countries – Sierra Leone, Liberia and Guinea.

Ms Cafferkey did not suffer from the usual fever and vomiting associated with the disease, which means it is possible people believed to have recovered from the disease have died months later from Ebola-related complications unnoticed, The Guardian reports.

Nottingham University professor of molecular virology Jonathan Ball told the publication there are plenty of unknowns when it comes to Ebola, “but I don’t think anybody would for one minute have expected complications quite as serious as they certainly appear”.

“What we do know is that survivors of past outbreaks do experience long-term health issues and they can be quite debilitating,” he said.

“They can prevent people from working but they wouldn’t have been deemed to be life-threatening.”

It is believed the Ebola virus can hide out in pockets of survivors bodies that the immune system does not protect – such as fluid inside the eye, the prostate gland, the placenta, breast milk and the central nervous system.

It was also discovered that men carry the virus in their semen for at least nine months, which means survivors can infect others months later.

A Sierra Leonean woman died in March from Ebola after being infected by a recent sexual partner.

Prof Ball said Ms Cafferkey was not presenting with “classic Ebola symptoms” so should not have posed a risk to other people, but around 60 people who have been in contact with the nurse are being monitored and offered a vaccination.

World Health Organisation figures indicate the most deadly outbreak since the virus was identified in central Africa in 1976 has killed 11,312 of the 28,457 people infected since December 2013, the ABC reports.