The Morningside Medi-Clinic nurse infected by the arena virus or rat fever is in a serious condition in the hospital.
Her health took a bad turn at the weekend.
"The condition of the nursing sister has become serious as the disease progresses," Melinda Pelser, hospital spokesman, said.
So far four people have been diagnosed with the rodent-borne virus in South Africa. All the cases were reported in Morningside Medi-Clinic in Johannesburg.
"The nurse in isolation was confirmed to be infected with the virus on Sunday last week.
"She contracted the virus from one of the dead people.
"We have traced people she came into direct contact with and they are being monitored," Pelser said.
Nearly 200 people who had direct contact with the four deceased are being monitored daily every six hours in Morningside Medi-Clinic and the Sir Albert Robinson Hospital on the West Rand.
They are being observed to see if they develop any of the disease's symptoms which include vomiting, diarrhoea, fever, headache and flu.
The process will continue for 21 days.
Meanwhile, the Johannesburg radiographer who was admitted to St Augustine's Hospital in Durban was discharged yesterday.
She had presented herself at the hospital with arena virus symptoms on Tuesday last week.
Several tests, however, suggested that the radiographer was not infected. She was placed in isolation while further test were conducted.