More than 100 people in South Africa are under medical observation after coming into contact with people who died from suspected haemorrhagic fever.
Doctors have tried to calm fears that the disease could spread throughout the wider population in Johannesburg.
The initial patient was a woman from Zambia who died in mid-September, two days after arriving in South Africa.
A paramedic who accompanied her and a nurse who treated her have also since died with similar symptoms.
These included fever, nausea and external bleeding, which indicts that is a haemorrhagic fever, but a definitive diagnosis is yet to be made.
"The public at large are not at risk," South Africa's Mail and Guardian newspaper quotes intensive care specialist professor Guy Richards as saying.
Those under observation will have their temperatures monitored every six hours for the next 21 days.
Meanwhile, the UN World Health Organization has been brought in to help health authorities, the South African Press Association reports.
"We are helping with the investigation to identify the cause of the disease and have flown in two experts," WHO spokesperson Dr James Mwanzia said.
He said the world health body was also working with the Zambian authorities.
Ebola and a few other haemorrhagic fevers have been responsible for a tiny number of deaths compared to Aids in Africa.
But the devastating speed at which they strike, and the far higher possibility of transmission from human to human have made the thought of a major outbreak a terrifying prospect.
Zambian health ministry official Simon Miti told the Mail and Guardian that no other cases had been reported in Zambia.