Legionnaires' disease is believed to be the cause of a pneumonia outbreak that killed four people in Argentina.
Eleven cases have so far been reported, linked to a private health clinic in the north-western city of San Miguel de Tucuman.
The cause had been a mystery until now, with COVID ruled out and other tests coming back negative. The World Health Organisation said on Friday that it was monitoring the situation.
Symptoms have included pneumonia in the lungs, shortness of breath, fever, and muscle and abdominal pains.
Argentinian health authorities said on Saturday that Legionnaires was the suspected cause. Tests had earlier ruled it out.
The Legionella bacteria is found naturally in freshwater environments but can grow and spread in building water systems, according to the US Centers of Disease Control and Prevention.
It is often transmitted when people inhale tiny contaminated droplets in the air, or sometimes from water containing the bacteria.
Health minister Carla Vizzotti told a news conference that people over the age of 50 with certain conditions can be particularly at risk.
Current or former smokers and those with lung problems are also more likely to get sick.
Legionnaires' disease was discovered and named in 1976 after an outbreak at a Philadelphia convention of the American Legion of veterans.