In Union County, five people have died after 22 contracted Legionnaire's disease, state health officials said Friday.

The outbreak has led authorities to ask county residents and workers who come down with pneumonia-like symptoms to seek prompt medical attention.

The cases were reported between March 8 and May 13. Patients who died were described as "older adults" who had significant medical conditions, the state Department of Health said.

Health officials would not identify specific communities where the cases were reported.

"This is an ongoing and complex investigation and at this point in the investigation the Department has not identified a source so we cannot narrow the geographic area beyond Union County at this time," spokeswoman Donna Leusner said.

Legionnaires’ disease is a type of pneumonia or lung infection caused by bacteria called Legionella, which occurs naturally in fresh-water environments. It can become a health concern when it grows and spreads in building water systems.

A case of Legionnaire's disease was reported in a resident of the Alvin E. Gershen Apartments in Hamilton. Legionnaire's disease was found in eight schools in the West Orange school district in September.

People can get sick with Legionnaire's when they breathe contaminated mist or contaminated water gets accidentally swallowed into the lungs. Most people exposed to legionella do not get sick. People older than 50, anyone who has ever been a smoker, and people with a weakened immune system or chronic disease are at increased risk.

The Centers for Disease Control and Prevention said about 1 in 10 people who get sick from Legionnaire's will die.

Health Commissioner Shereef Elnahal said the risk to any recent visitor to Union County is very small but recommended that individuals who live there who became ill with pneumonia-like/respiratory symptoms, such as fever, chills, cough, shortness of breath, muscle aches, and headache visit their healthcare provider "out of an abundance of caution."

Symptoms can take two weeks to develop.

Contact reporter Dan Alexander at Dan.Alexander@townsquaremedia.com or via Twitter @DanAlexanderNJ

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