Cyclospora can not be transmitted from one person to another, but only through consuming food or water infected with feces that contain the parasite. About one quarter of the cases have been linked to McDonald's salads.
"Although a link has been made to salads sold in McDonald's restaurants in some IL cases, public-health officials continue to investigate other sources", IL health director Dr. Nirav Shah, said in a statement.
IL has reported 90 cases since mid-May.More news: Air China jet plunged 25,000ft because pilot was vaping in cockpit
Officials in Iowa are asking the same of anyone who has developed similar symptoms since mid-June, saying all 15 of the recorded cases there have been linked to McDonald's salads.
The most common symptom of cyclosporiasis is watery diarrhea, some other ones include bloating and gas, fatigue, stomach cramps, weight loss, mild fevers and nausea.
The departments of public health in IL and Iowa both announced Thursday that an investigation into the outbreak is underway.
Cyclospora is a parasite commonly found in developing countries.More news: I didn't criticise Theresa May to The Sun
The illness can last from a few days to a few months and patients might feel better, then get worse again. Healthcare providers usually treat cyclosporiasis using antibiotics.
Those who have the illness need prescription medicine to recover, according to the public health agency, and should start feeling better about two to three days after treatment begins.
"Out of an abundance of caution, we made a decision to voluntarily stop selling salads at impacted restaurants until we can switch to another lettuce-blend supplier".
McDonald's is in the process of removing potentially contaminated salads from its restaurants, and re-supplying them.More news: Justice Department to appeal approval of AT&T acquisition of Time Warner
Officials say the fast-food company has been fully cooperating with the investigation and McDonald's and involved federal partners are determining what further steps should be taken.