The eight cases were among 8,500 people staying at the JW Marriott between July 16 and August 16. Marriott officials said Tuesday they have been able to contact 80 percent of the guests who stayed at the downtown hotel during that time to alert them of the possible risk.

There is "no ongoing health threat at the hotel," according to Dr. Kathleen Ritger, Medical Director over Communicable Disease at the Chicago Department of Public Health.

Legionnaires' disease is a severe form of pneumonia. The disease is contracted by breathing in a mist or vapor contaminated with the Legionella bacteria, which can grow in cooling towers, showers and other water sources. People already in poor health are the most vulnerable.

Another recent outbreak has caused the death of eight people in the Canadian province of Quebec. Canadian officials say they have identified more than 100 cases since July.

The illness is named for a 1976 outbreak at an American Legion convention in Philadelphia. An average of 30 cases are reported per year in Chicago. (Reporting By Mary Wisniewski; editing by Andrew Hay)

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