A woman who died in Sierra Leone on January 12 has tested positive for the Ebola virus, the World Health Organization (WHO) said on Friday, one day after the UN agency declared an end to virus' outbreak in West Africa.

A spokeswoman for the National Ebola Response Centre in Sierra Leone, Tunis Yahyam, confirmed that a swab tested positive for Ebola after the young lady died in Magburaka, the largest city of Tonkolili district in the Northern Province of Sierra Leone.

The new case shows that the virus still poses a risk and that health professionals would remain in the region, WHO spokesman Tarik Jasarevic told a news briefing.

"The main message is that the job is not done yet," Jasarevic said, adding that the woman's contacts must still be traced and quarantined.

Health authorities will then need to monitor those connected to the woman for a 42-day incubation period to see if any other Ebola cases develop. 

The chance of further infections will be eliminated 42 days after the last confirmed infected case produces two negative blood tests for the virus.

After the 42-day period, a country is still expected to maintain heightened surveillance for an additional 90 days.

The Ebola outbreak in West Africa, which started in Guinea in December 2013, was the largest in history, spreading to Sierra Leone, Libiera and Nigeria.

Sierra Leone was declared free of Ebola transmission on November 7, Guinea on December 29 and Liberia on January 14.

It also reached Europe and the United States, with isolated cases of the disease reported.

The WHO declared the region free of Ebola 42 days after the last case was confirmed in Liberia, the final of the three West African countries with active transmission of the virus.

WHO experts in Geneva warned during Thursday's announcement that there was still a risk that the hemorrhagic fever could flare up again, as the virus can persist for up to 12 months in the semen of some male survivors.

More than 11,300 people have died since the outbreak in December 2013, while 28,500 have been infected, according to the WHO's latest figures.

Ebola, which was discovered in 1976 and is passed on through contact with blood and other bodily fluids, causes massive hemorrhaging and has a fatality rate of up to 90 per cent if left untreated. 

Related stories

New Ebola case reported in Sierra Leone

Second person tests positive for Ebola in Sierra Leone

WHO declares West Africa Ebola-free

Liberia closes border with Guinea after Ebola "flare up"

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