More than 20,000 people have been infected by Ebola in Sierra Leone, Liberia and Guinea since the outbreak of the virus began, the World Health Organisation has said.

The WHO said in a statement on Monday that there have been 20,081 cumulative cases of infection in the three worst hit countries, and more than 7,842 Ebola-related deaths recorded.

The virus is still spreading intensely in Sierra Leone, the organisation said, with 315 new confirmed cases reported in the week to December 21. These included 115 cases in the capital Freetown.

In Guinea, 156 confirmed cases were recorded during the same period, "the highest weekly case incidence reported by the country in this outbreak", it said.

In Liberia, where case incidence has been declining for the past month, 21 cases were reported in the week to December 21.

Five additional countries - Nigeria, Senegal, Mali, Spain and the US - have had imported cases and are included in the global toll.

UK case

A healthcare worker recently back from Sierra Leone was diagnosed with Ebola on Monday by doctors in Scotland's largest city, the first diagnosis of the deadly virus in Britain during the current outbreak.

Scottish First Minister Nicola Sturgeon confirmed that the patient was a female health worker who had been working on the "frontline" with Ebola patients, and was currently in a stable condition in hospital.

Save the Children later confirmed she was a National Health Service employee working for the charity.

The patient returned to Scotland late on Sunday via Casablanca and London Heathrow, arriving at Glasgow Airport at around 11:30pm (2330 GMT), according to a Scottish government press release.

Bodily fluids

Ebola spreads through direct contact with the bodily fluids of an infected person showing symptoms such as fever or vomiting.

People caring for the sick or handling the bodies of people infected Ebola are especially exposed.

As of December 21 a total of 666 healthcare workers were known to have contracted the virus, and 366 of them had died, according to the WHO.

The Ebola epidemic, which claimed its first victim in Guinea exactly a year ago, is likely to last until the end of 2015, according to Peter Piot, a scientist who helped to discover the virus in 1976.

No cure or vaccine is currently available for Ebola, with the WHO authorising the use of largely untested treatments in efforts to combat the disease.