An outbreak of Legionnaires’ disease at an apartment complex in Portland, Ore., has killed at least one person and hospitalized four others, officials said this week. 

The Multnomah County Health Department in a Tuesday news release said more than 100 residents of Rosemont Court in North Portland have been urged to temporarily relocate after multiple people became ill with pneumonia. 

Investigations on Monday revealed the building’s water supply is the likely source of the Legionnaires’ outbreak.

The health department is now "working with Northwest Housing Alternatives which owns Rosemont Court to temporarily relocate residents and ensure proper cleaning of the building plumbing system to remove any remaining Legionella bacteria," which is naturally found in freshwater, "but can cause health problems if it gets into a building’s water supply," health officials said. 

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Legionnaires’ disease is a severe form of pneumonia. People contract the disease when inhaling Legionella bacteria. The illness is not spread person-to-person.

Legionella pneumophila, a bacterium, is usually the cause of the illness, per the Mayo Clinic. It can be found in soil and water, but more commonly causes infection when it multiplies in water systems such as hot tubs and air conditioners.

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"This is a unique and concerning situation and we are working with our partners as quickly as we can to determine if other properties or water sources are affected," said Dr. Jennifer Vines, Multnomah County Health Officer, in a statement. 

Residents have been offered temporary housing at local motels, with 20 residents choosing to leave the building on Monday night and others expected to follow suit, according to the health department.