A government source in Sierra Leone said the World Health Organisation worker who contracted Ebola in the country was a Senegalese expert working for the WHO in the eastern town of Kailahun, Reuters news agency reported.
The WHO said it was the first time one of its healthcare workers had tested positive for the deadly virus.
More than 160 staff will work in a UK-funded facility in Sierra Leone which is due to open in October.
UK trials of a vaccine for the Ebola virus began today - but some experts complain that the response has been a long time coming.
A Swansea doctor who took leave to treat Ebola patients in Africa, is back on the wards after three weeks in isolation.