While not from Illinois, I have heard about this previously.
I am curious how this may connect with the alleged great die-off that occurred just prior to European colonial migration beginning.
As the die-off of Cahokia precedes Columbus' landing in the New World which caused a die-off in Central and South America, it can't be blamed on the diseases the Spaniards brought over, but the time frame matches the die-off caused by the diseases the Vikings brought to Newfoundland.
The “great die-off” was due to smallpox (and probably some other diseases that the natives had no resistance to). The Spanish brought their bugs with them to Mexico and South America, and within a few years, the diseases spread all throughout North America too.