Three people have died of an unknown pneumonia in Argentina, with COVID and dozens of other illnesses ruled out

• An unknown pneumonia has killed three people and sickened six others in Argentina.
• Tests for COVID-19, flu, and other pneumonia-causing bugs were negative, health authorities said.

A pneumonia of unknown origin has killed three people and sickened six others in Argentina, with tests for pneumonia-causing bugs, including COVID-19, coming back negative, according to the country's health authorities.

All of the cases have been reported in Tucumán — a small region in the northwest of Argentina, around 800 miles from the capital of Buenos Aires — since late August, according to the Pan American Health Organization (PAHO).

Eight of the cases were health workers, and the other was a patient from an intensive care unit in a private clinic in the city of San Miguel de Tucumán, it said.

As of Thursday, three people had died, and all of them had underlying health conditions, it said.

Professor Devi Sridhar, chair of global health at Edinburgh University, UK, and author of "Preventable," told The Telegraph: "It's obviously concerning but we still need key information on transmission and hopefully [on the] underlying cause."

She said: "This shows our collective vulnerability to dangerous pathogens. An outbreak in any part of the world – if not quickly contained – can spread rapidly given air travel and trade."

Symptoms include fevers, muscle pain, and breathing problems

The first six cases got symptoms — including fevers, muscle pain, abdominal pain, and difficulty breathing — between August 18 and August 22, according to Ministry of Public Health of Tucumán Province.

Two of these patients died, three were hospitalized, and the other was in "good health" in isolation at home, it said.

Three further cases — all health workers — developed similar symptoms between August 20 and 23, PAHO said.

Luis Medina Ruiz, Tucumán's minister of health, said on Wednesday the patients all had a "severe respiratory condition," and pneumonia in both lungs that looked like COVID-19 on a chest x-ray, per The Telegraph.

He said that the six unwell patients had been tested for COVID-19, common flu viruses, and 26 other bugs, but nothing had been identified.

Additional investigations, including toxicology tests, were ongoing, PAHO said.

The cause of the pneumonia is unclear

Jonathan Ball, a professor of virology at the University of Nottingham, UK, told The Telegraph that the illness could be a "rare event" caused by a known respiratory virus that hadn't been tested for, or something new. Genetic sequencing can distinguish between the two, he said.

As COVID-19 restrictions that curb other respiratory viruses ease, Ball said that experts should be on alert for unusual symptoms.

"[This] might not be new illnesses – just a cluster of rare events from known illnesses," he said. "Expect more until we settle back into the normal seasonal ebb and flow of respiratory viruses."