Last year, an outbreak of Legionnaires’ disease at UWMC sickened five people, including two who died.

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Patients at the University of Washington Medical Center (UWMC) have again been diagnosed with Legionnaires’ disease believed to have been contracted at the hospital, including a woman in her 20s who died Friday.

Last year, an outbreak at UWMC sickened five people, including two who died.

In the new round of cases, occurring in the same building as last year, UWMC reported Friday morning that a man his 40s was in the process of being discharged. His case was reported to the health agency Wednesday.

The woman’s case was reported to Public Health – Seattle & King County on Thursday. The agency said in a news release Friday morning that the woman was “very ill, had multiple underlying conditions, and it is not yet known whether Legionella was a cause of death.”

The patients’ medical histories suggest exposure to Legionella, a severe type of pneumonia, occurred while they were hospitalized at UWMC, according to a Public Health news release.

Legionnaires’ disease stems from an organism widely found in natural water supplies, and it occurs when people breathe in mist or vapor contaminated with the bacteria. Healthy people exposed to the bacteria typically don’t get sick, but it can be dangerous, even deadly, to people older than 50, former or current smokers and particularly those with weakened immune systems.

“It is a very challenging and difficult organism to eradicate from building water systems,” Dr. Jeffrey Duchin, health officer at Public Health, said at a news briefing Friday.

He said the outbreak doesn’t represent a public health risk and that the disease is rarely spread person to person.

A third patient, a man in his 60s whose case also was reported to Public Health on Thursday, has been diagnosed with Legionella pneumonia, but he appears to have been infected in the community and not at UWMC, officials said. He remained hospitalized in stable condition.

He and the other man were being treated with antibiotics, Dr. Tom Staiger, medical director at UWMC, said at the news briefing.

Cases typically occur two to 10 days after exposure to the bacteria.

Last year, a Legionella infection was detected during an autopsy of a woman in her 50s who had been a UWMC patient. A man in his 50s also died.

That outbreak was traced to the water system of Cascade Tower, the building also affected this year, according to UWMC. All known risk factors that may have contributed to the 2016 cases were mitigated, UWMC said.

Crews installed filters last year, flushed water systems and took other steps at the hospital to prevent the spread of the bacteria. For a while, signs warned patients not to use drinking fountains and sinks. Coffee stands and other vending areas were shuttered.

Sinks and ice machines were among places the bacteria were found last year, and were being checked anew.

Public Health reported Friday that UWMC is again working to protect patients, visitors and employees by limiting exposure to water sources, including tap and shower water, while they investigate. Bottled water is being used for drinking, and filters for bacteria were being fitted on sinks and showers.

UWMC voluntarily adopted a “very aggressive water-management program” last year, based on guidance from the Centers for Disease Control and Prevention, Duchin said.

He said an examination was underway to determine if additional steps are needed to deal with a complex problem that goes beyond UWMC.

Staiger said extensive monitoring and testing was implemented after last year’s outbreak. Water is monitored every 10 minutes, with an alarm system, he said.

Nearly 500 water-sample tests in the last year have not suggested a systemic issue with Legionella in the supply, Staiger said.

As recently as early July, “very, very” low levels of the bacteria were found in four of roughly 35 cultures from sites in Cascade Tower that were cleaned, he said.

Levels have been dramatically lower than what was seen last year, Staiger said, adding officials don’t believe a spike occurred since early July, but are checking to see if that happened.

When low levels have been detected, corrective action has been taken, UWMC said.

Staiger said the precise source of the current outbreak has not been identified, but the medical center anticipates being able to answer that question and, in consultation with Public Health and outside experts, deal with the problem.

“Patients and staff safety are our utmost concern,” Staiger said, including some high-risk patients who might be put on antibiotics for preventive purposes.

Staiger said he wasn’t aware of anything that would make Cascade Tower more susceptible to infections.

About 5,000 cases of Legionnaires’ disease are reported a year in the U.S. This state saw 72 cases, including 10 deaths, in 2016. The disease was first identified after an outbreak in 1976 sickened many at an American Legion meeting in Philadelphia.