Health officials in McHenry County are investigating several cases of Legionnaires' disease that occurred between June 7 and July 1.
Nine people became ill at different locations across the county.
The victims range in age from 46 to 82.
None of them lived in a public facility.
Department spokeswoman Keri Zaleski said Wednesday the source of the outbreak and a common denominator hasn't been found.
The Illinois Department of Public Health is assisting in the investigation.
According to a news release from the McHenry County Department of Health, Legionnaires' disease is caused by waterborne bacteria inhaled from vapor.
The disease occurs more frequently in hot and humid weather, and is caused by a type of bacteria commonly found in the environment.
It can become a health concern when it is found in building water systems such as shower heads, hot tubs, fountains, hot water tanks and large plumbing systems, the news release reads.
Symptoms often start two to 10 days after exposure and include diarrhea, nausea, vomiting, aching muscles, fever and chills.
"Legionnaires' disease is not known to spread person to person," Illinois Department of Public Health Chief Medical Officer Jennifer Layden said. "Most healthy people do not get Legionnaires' disease after being exposed to Legionella bacteria. Individuals at increased risk of developing Legionnaires' disease include those older than 50 years of age, or who have certain risk factors, such as being a current or former smoker, having a chronic disease, or having a weakened immune system."
People should see their health care provider if they are symptomatic after possibly being exposed to Legionella bacteria, officials said.
A 2015 outbreak at the Quincy veterans home in western Illinois caused the death of 13 residents and sickened dozens more.
The McHenry County Department of Health is working with the Illinois Department of Public Health to investigate these occurrences.
For more information, people should visit cdc.gov/legionella.