The organisation's spokeswoman Fadela Chaib also told journalists in Geneva that 121 of those are already under observation, and that "all are doing well."
WHO epidemologists have been been hunting for the cause of the mysterious illness which killed "an expatriate tour guide living in Lusaka" as well as a paramedic and a nurse, she added.
The disease is believed to be a haemorrhagic fever, but "three haemorrhagic fevers have been ruled out - Ebola, Rift Valley and Lhasa. They are not the cause of the disease," said Chaib.
The first victim arrived in South Africa on September 12 and died two days later after being treated for what was believed to be tick-bite fever. The medic who accompanied her from Zambia died two days later with flu-like symptoms.
A nurse has also died since.