A third person has died this week in Argentina from a type of pneumonia of unknown origin, with the fatalities thus far limited to a single clinic, health authorities said Thursday.

Nine people in northwestern Tucuman province have come down with a mysterious respiratory illness, including eight medical staff at the private clinic, Tucuman health minister Luis Medina Ruiz told reporters.

Three people — two health care workers and now also a patient at the clinic — have died since Monday.

The Pan American Health Organization (PAHO), the regional office of the World Health Organization, is in contact with the Argentina Ministry of Health as they investigate the cluster of cases, PAHO said in a statement. 

Authorities are conducting tests but Medina said they have already ruled out COVID-19, flu, influenza types A and B, the legionella bacterial disease and the hantavirus spread by rodents.

Samples have been sent to the Malbran Institute in Buenos Aires.

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

Medina said the woman could have been “patient zero, but that is being evaluated.”

The mystery disease claimed its first victim among health personnel at the clinic on Monday and a second two days later.

The first six patients started exhibiting symptoms between Aug. 18 and 23.

Medina said on Wednesday the patients were struck with “a severe respiratory condition with bilateral pneumonia… very similar to Covid.”

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

Of the six people receiving treatment, four were in serious condition in the hospital and two were in isolation at home.

All other staff at the clinic were being monitored.

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

The provincial health ministry said Wednesday the outbreak could have come from an infectious agent, but investigators were not excluding “toxic or environmental causes.”

Infectious disease specialist Mario Raya said Thursday that “for the moment, we have no cases outside” the stricken clinic.

Added Hector Sale, president of the Tucuman provincial medical college: “We are not dealing with a disease that causes person-to-person transmission” as no cases have been identified among close contacts of any of the patients.

Dr. Michael Osterholm, an infectious disease expert at the University of Minnesota, told Reuters that since the lungs are heavily involved, the cause is likely something the patients inhaled.

Osterholm said “mystery illnesses” do sometimes happen, and most often they can be explained by some local outbreak that does not have pandemic implications.