SAN JOSE, Calif. — Santa Clara County Public Health is investigating an outbreak of legionella at Kaiser Permanente’s Santa Clara Medical Center, where at least 18 people were infected with the bacteria that spreads in water systems.

Most people exposed to legionella don’t get sick, and risk to the broader South Bay community is “low,” a spokesperson for the Santa Clara County Public Health Department said in an email.

“Public Health is working with the facility to ensure anyone with symptoms is appropriately treated and that the facility is taking necessary steps to prevent people from getting sick,” the spokesperson said.

A spokesperson for Kaiser said the Oakland-based healthcare giant has not located the source of the bacteria. It grows and spreads in showerheads, sink faucets and “large, complex plumbing systems,” according to the Centers for Disease Control and Prevention. People typically get exposed when they inhale water droplets containing Legionella.

Kaiser officials have not said if the exposed people were staff, patients or visitors to the medical center. It is operating as usual, the spokesperson said.

“While we work to determine the source, out of an abundance of caution, on Monday we concluded additional water treatment measures and preventive measures to ensure the safety of all patients, employees, and visitors,” the spokesperson said in an email. “As a result of mitigation efforts, our hospital and medical office building continue to operate normally and continue to care safely for patients.”

Although most people exposed do not get sick, the bacteria cause Legionnaires’ Disease, a serious type of pneumonia. Symptoms include shortness of breath, chest pain and aching muscles. Older people and people with weak immune systems are more at risk of developing Legionnaires’ Disease, the CDC said.

The fatality rate of Legionnaires’ Disease is 10%, according to the CDC. That rises to 25% in cases at healthcare facilities.

“Most healthy people exposed to Legionella don’t get sick,” the Health Department spokesperson said. “Legionnaires’ disease is treated with antibiotics and most people who get sick make a full recovery.”

The CDC recommends that hospitals keep protocols for preventing and responding to legionella outbreaks.

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