The World Health Organization, a U.N. health agency, said Friday that it is investigating a mysterious disease that has so far killed three people in the South African city of Johannesburg. WHO says the disease appears to be a form of hemorrhagic fever.

Tests have proved negative for Ebola, Lassa fever, Rift Valley fever, Marburg fever and other main types of hemorrhagic fever.

Gregory Hartl, a WHO spokesman, said that the first death on Septemer 13 was a female tour guide who had fall ill in Zambia before being evacuated to South Africa. The subsequent two deaths on September 30 and October 4 involved a paramedic and a nurse who treated the woman.

WHO said Friday that 121 people are currently being monitored, and that they hope to receive further test results by Sunday.