"One hundred twenty-one known contacts of the fatal cases are being traced in South Africa and 23 in Zambia," the World Health Organization said in a statement.

The South African cases are "all are doing well," a spokeswoman said. She said she had no immediate word on the Zambian cases.

Agency epidemiologists believe the disease is a viral hemorrhagic fever, one of a diverse group of illnesses caused by five distinct families of RNA viruses, which are viruses that have ribonucleic acid as their genetic material.

Notable human diseases caused by RNA viruses include SARS, influenza and hepatitis C.

The first death from the mystery illness was a tour guide who became ill in Zambia Sept. 12 before being evacuated to South Africa for what was believed to be South African tick-bite fever, said WHO, a coordinating authority on international public health.

Two other deaths Sept. 30 and last Saturday involved a paramedic and a nurse who treated the tour guide.

WHO said it hoped to have further test results Sunday.