Health authorities in Santa Clara County are investigating an outbreak of Legionella infections connected to Kaiser Permanente Santa Clara.

Officials have confirmed 18 cases so far, prompting a coordinated response between the hospital and public health experts.

Kaiser Permanente said the bacteria were first detected during a routine internal monitoring process.

The health system shared that it remains uncertain whether the individuals who contracted the infection were patients, staff members, or visitors.

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In a statement, the hospital explained that “Legionella bacteria occur naturally in water and are spread by inhaling contaminated mist, not through person-to-person contact.”

Officials added that, while the investigation continues, additional water treatment and preventive measures are underway to protect patients, employees, and visitors.

Despite the confirmed cases, the hospital remains open. Medical staff continue providing care while the source of contamination is traced.

Legionella bacteria can occur in both natural and human-made water environments, including lakes, rivers, faucets, showerheads, hot tubs, and decorative fountains. These bacteria thrive in warm water and can spread through airborne water droplets.

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Dr. Peter Chin-Hong, an infectious disease specialist at UCSF, said that people can become infected by breathing in mist from contaminated systems. “The bacteria can cause a whole host of things from very mild to serious,” he explained.

“When it’s serious, it’s called Legionnaire’s disease. Usually you get pneumonia, but you can also get nausea, vomiting and diarrhea.”

The California Department of Public Health notes that small droplets of water from contaminated systems can lead to two types of illnesses: Legionnaires’ disease, which causes pneumonia-like symptoms, and Pontiac fever, a milder condition.

“It can be deadly, but in the order of 10-30% in those who are most seriously at risk,” Dr. Chin-Hong said.

According to county and state health officials, Legionella infections do not spread from person to person. Most healthy individuals who are exposed to the bacteria do not become ill.

The hospital confirmed that several people had been infected and reiterated that the bacteria cannot spread between individuals. Experts are focused on identifying exactly where in the facility’s water system the contamination originated.

Legionnaires’ disease may resemble other forms of pneumonia, and those with underlying health conditions are more susceptible to serious illness.

People at higher risk include those with chronic lung disease such as COPD and emphysema, cancer, diabetes, kidney or liver failure, or anyone taking medications that weaken the immune system, including chemotherapy or post-transplant treatments.

Public health leaders said efforts to pinpoint the bacterial source and ensure continued safety remain ongoing.

They added that the hospital’s enhanced water treatment measures are part of a broader strategy to prevent further infections as patient care continues uninterrupted.