The World Health Organization on Sunday announced the first Ebola infection among its experts, describing the infected person as an epidemiologist who had been deployed to Sierra Leone.
The WHO did not provide information about the sex, age or nationality of the person but said the patient was "receiving the best care possible".
The UN organisation said it was considering "the option of medical evacuation to another care facility if necessary".
Since the outbreak of Ebola in West Africa this year, the WHO has sent 400 people to help battle the epidemic.
More than 225 health workers have contracted the contagious tropical disease while treating people in Guinea, Liberia, Sierra Leone and Nigeria, and 130 of them have died, the WHO said.