Neither patient was identified by name, and the nationality of the infected WHO employee was not given.
The British patient was working at an Ebola treatment center in eastern Sierra Leone, the region most affected by the outbreak, said Sidie Yayah Tunis, director of communications for the Sierra Leone health ministry.
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The two cases highlight the risks facing health workers on the front lines of the battle against Ebola, which has killed more than 1,400 people across West Africa, according to the latest WHO figures.
"This is the first time someone working under the aegis of WHO has fallen ill with the disease," the WHO said in its statement, adding that more than 225 health workers have been infected and nearly 130 have died from Ebola during the current outbreak.
The British patient was transported via ambulance to Sierra Leone's main airport in the town of Lungi, Tunis said.
Britain's Department of Health said the patient was being flown on a specially equipped RAF transport plane to Northolt air base in London.
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He will be treated at London's Royal Free Hospital, which has an isolation unit for infectious disease.
The department said in a statement that the patient "is not currently seriously unwell."
The World Health Organization says Sierra Leone has recorded 910 Ebola cases and 392 deaths. The Sierra Leone government says there have been 881 cases and 333 deaths. In Kenema, where the Briton was working, the government has recorded 303 cases.
Two Americans and a Spanish medical worker have already been evacuated from Liberia and given ZMapp, an experimental and unproven treatment for Ebola. The Americans have recovered and been discharged while the Spaniard died.
The drug supply is now exhausted, the U.S. manufacturer has said.