A third patient died this week from pneumonia of unknown cause in Argentina. Deaths have so far been confined to her single clinic, health officials said Thursday.

Health Minister Luis Medina Ruiz of northwestern Tucuman province told reporters that nine people, including eight medical staff at a private clinic, had a mysterious respiratory illness.

Two health care workers and three people who are now clinic patients have died since Monday.

The Pan American Health Organization (PAHO), the regional office of the World Health Organization, has been in contact with the Argentine Ministry of Health and is investigating the string of cases, PAHO said in a statement.

Authorities are conducting tests, but Medina said it had already ruled out COVID-19, influenza, influenza A and B, Legionella disease and hantavirus spread by rodents. .

The samples were sent to Malblanc Laboratories in Buenos Aires.

The latest victim was a 70-year-old woman who was admitted to the clinic for surgery.

Medina said the woman “could have been patient zero, but it’s being evaluated.”

The mysterious illness claimed its first casualty among health care workers at the clinic on Monday, followed by a second victim two days later.

The first six patients began showing symptoms between August 18 and 23.

Medina said Wednesday that the patient was struck with a “severe respiratory condition with bilateral pneumonia… very similar to Covid.”

Symptoms included vomiting, high fever, diarrhea, and body aches.

Of the six people being treated, four were in critical condition in hospital and two were in isolation at home.

All other staff at the clinic were monitored.

Experts were analyzing water and air conditioners for possible contamination and poisoning.

The state’s Department of Health said Wednesday that the outbreak could have originated from an infectious agent, but investigators had not ruled out “toxic or environmental causes.”

“For the time being, there are no cases outside of the affected clinics,” infectious disease expert Mario Raya said on Thursday.

“We are not dealing with diseases that cause human-to-human transmission,” added Hector Sale, president of Tucuman State Medical College.

Dr. Michael Osterholm, an infectious disease specialist at the University of Minnesota, told Reuters the cause was likely inhaled by the patient, as the lungs are heavily involved.

Osterholm said “mysterious illnesses” can also occur and can be mostly explained by localized outbreaks unrelated to the pandemic.