A woman in Liberia is feared to have Ebola after having unprotected sex with a survivor of the deadly disease, more than six months after he was cured.

The 44-year-old woman from Monrovia, tested positively for Ebola on March 20, 14 days after having unprotected sex with a 46-year-old, who had previously tested negative for the lethal virus.

According to a report from the Centres for Disease Control and Prevention, the man was initially infected with Ebola on September 9, 2014. He was released from the treatment centre October 7 after his symptoms cleared up and several tests showed no sign of the virus in his bloodstream.

However, it is thought that traces of the virus survived in his semen, meaning he could have infected the woman after having unprotected sex with her on March 7 this year.

The woman, who is currently the only confirmed case in the region, first began showing signs of the virus seven days later, was admitted to hospital on March 17, sent to an isolation two days later, and was officially diagnosed on March 20.

If the transmission is confirmed it will mean that the virus survived in his semen for 199 days. The previous limit for the virus surviving was thought to be 82 days.

The case suggests patients who are deemed cured of the virus, in fact stay infectious for more than twice as long as previously thought.

The worrying finding raises fears that eradicating the virus could be much more difficult than previously thought.

While medics say they cannot confirm that the disease was passed on through sex, they have advised all survivors to use condoms every time they have intercourse.

Given the latest evidence in the Liberian survivor's case, the CDC joined the WHO and Liberia in issuing the guidelines on safe sex.

According to the World Health Organization, 26,298 people have been infected with Ebola in the West African outbreak, and 10,892 have died, mostly in Liberia, Guinea and Sierra Leone.

Liberia has had no new Ebola cases in five weeks. That country's last confirmed case died on March 27. The WHO declares countries to be Ebola-free 42 days after the last known case. If there are no additional cases, Liberia could be declared Ebola-free May 9.

The deadly virus is contracted by contact with infected bodily fluids and its symptoms include fever, diarrhoea, vomiting, stomach and chest pain.