Just a 10-­minute walk separates UPMC Presbyterian from the Veterans Affairs Pittsburgh Healthcare System in Oakland, the hubs of two clusters of Legionnaires’ disease during the past three years.

But confirmation this week of Legionnaires’ cases at Presby should not raise red flags over the neighborhood water supply, a leading microbiologist said. Tainted ice machines blamed in the Presby cases are a rare source for the disease, caused when immune-­compromised people inhale common Legionella bacteria from contaminated water or mist, said Janet E. Stout, director at the Uptown-­based Special Pathogens Laboratory.

The Tribune-­Review reported Thursday on the Presby incident, in which one patient died and two fell ill last year after chewing on the hospital’s ice chips. UPMC officials are sterilizing about 500 ice machines throughout its 20-­hospital network, including 90 at Presby.

Six veterans died and 16 were sickened from February 2011 to November 2012 in the VA outbreak, which federal reviewers linked to bacteria-­contaminated tap water and poor management of internal water systems. “The concept of eliminating Legionella from every nook and cranny — it’s not really something that can be achieved,” Stout said. She said ice machines are a much lower risk than hospitals’ hot-­water systems, where Legionella can multiply on a larger scale and reach more patients.

Stout wrote in 1985 about the presence of Legionella in hospital ice machines [Stout J, Yu VL, Muraca P. Isolation of Legionella pneumophila from the cold water of hospital ice machines: implications for origin and transmission of the organism. Infect Control 1985;6:141-146]. Since then, federal guidelines and hospital policies for Legionella control have focused on safeguarding hot-water systems from Legionella through specialized ionization systems and chemical treatments.