It got its name in 1976 when it struck participants at an American Legion convention in Philadelphia. Legionnaire's Disease hospitalizes between 8,000 to 18,000 people each year in the United States.
While most people recover, Legionnaire's Disease can be fatal.
The cases appeared from mid-September through early October. The health department called it a cluster, of six people, all relatively close together in the south suburbs.
Where they developed the disease, however, is still a mystery. It can affect just about anyone who comes in contact with the bacteria that causes Legionnaires' Disease. Over the years, doctors have determined it generally survives in water and becomes airborne.
"What is unusual is the clustering of cases we've seen in Tinley. So we focused the investigation to see whether or not these persons had anything in common, any common exposure to water that may have predisposed them to this infection," said Dr. Michael Vernon, Cook County Health Dept.
The six victims are all older than 50 and all are smokers - both factors that make them more vulnerable to the disease. All have been treated and released from local hospitals.
The health department had them answer an extensive questionnaire to try to determine what they have in common. So far, they've determined three shopped at the same grocery store and may have been exposed to the misters in the produce section. They've investigated that possibility.
"We've looked in the area for other possible sources of contamination to contaminated mist or vapor and we've not detected any cooling towers which may have contributed to it. If there were, it would be more widespread," said Vernon.
In June, McHenry County officials found two cases linked to a hotel hot tub. Neither was fatal.
Typically, there are about 15 cases of Legionnaires' Disease a year in suburban Cook County. So far, this year there have been 30.
Symptoms of Legionnaires' Disease include fever, headache and often a cough. It sometimes can develop into pneumonia. It can become fatal. However, antibiotics have become more effective at treating it once it's diagnosed.