The death toll from the Ebola virus has more than doubled to 31 in an area of Congo in recent days, according to the UN health agency.
The World Health Organisation said there have been 69 cases, including nine confirmed by a lab. Five of the deaths have been confirmed by lab work.
WHO spokeswoman Fadela Chaib told reporters in Geneva that "the situation is serious".
Adding to the mystery, she said, is that "it is unusual that the first person to be infected with Ebola was a health worker" and not someone working outside.
Authorities said last week that the outbreak had killed 15 people in the north-east of the Democratic Republic of Congo.
Ebola has no cure and is fatal in 40% to 90% of cases. The disease causes severe internal bleeding.
There have been eight previous Ebola epidemics in Congo since the first discovery of it in 1976. This is the first outbreak in the Haut-Uele territory, in north-eastern Democratic Republic of Congo.
The tradition of washing and displaying corpses before funerals can spread the epidemic. The ritual is intended to show love and respect for the deceased, but the practice also brings people into close contact with victims of the virus.
Aid group Doctors Without Borders, along with WHO and Congo's health ministry, have been conducting education campaigns in the area to warn people of the risks linked to this practice.
Doctors without Borders is managing two quarantine centres in Isiro and Viadana and the UN has called for the creation of a million-pound fund to fight the epidemic.