A third person died from a type of pneumonia of unknown origin in Argentina's Tucuman province, authorities said.

Tucuman Health Minister Luis Medina Ruiz told reporters that nine people were diagnosed with the mysterious respiratory illness, including eight medical staff at a private clinic.

So far, all the cases were connected to one health clinic. Two health workers and a patient at the clinic have died of the illness.

The latest fatality was a 70-year-old woman who had undergone surgery at the clinic. The victim is being treated "patient zero," Medina Ruiz said. In medical terms, "patient zero" is the first person to be infected with an illness or disease in a population during an outbreak.

Two people are currently hospitalized with mechanical respiratory assistance and in serious condition, according to the health minister.

'No proof of person-to-person transmission'The first symptoms of the mysterious illness were recorded between August 18 and 22.

The medical facility has been placed in preventive isolation for seven days as Argentinian health authorities study the origin and nature of the outbreak.

Even though the symptoms appear similar to COVID-19, epidemiological investigations have ruled out that illness, along with both influenza A and B, or hantavirus, spread by rodents. They have not yet excluded toxic or environmental causes.

Hector Sale, president of the Tucuman provincial medical college said that at this time there is no conclusive proof of person-to-person transmission as there have been no cases among those in close contacts of any of the patients.