An unknown disease has killed three persons in the South African capital of Johannesburg, the World Health Organization (WHO) said on Friday.

The first person who caught the disease was a woman living and working in Zambia. She was evacuated to South Africa and died in a Johannesburg hospital on Sept. 14, the UN agency said in a statement.

A paramedic and a nurse who cared for the patient also got infected and died in Johannesburg on Oct. 2 and Oct. 5 respectively.

Up to now there have been no further known symptomatic cases, either in Zambia or in South Africa. But 121 known contacts of the fatal cases are being traced in South Africa and 23 in Zambia.

Clinical features common to the three patients initially include fever, headache, diarrhea and myalgia developing into rash and hepatic dysfunction, followed by rapid deterioration and death, WHO said.

Laboratory analysis has been conducted in South Africa, and samples have, so far, tested negative for a series of viral haemorrhagic fevers and other common infectious disease pathogens.

Tests to identify the pathogen are now continuing in South Africa and further testing will be performed in the United States.

There is no indication at this point of the need for any restriction of travel to or from Zambia or South Africa and no special measures are required for passengers arriving from these countries, WHO said.