Several officials attending the NATO Summit became ill with what were described as "stomach troubles."
"A number of NATO officials have suffered from stomach troubles. Some are receiving medical care, others have already recovered," NATO press officials said.
However, they said the illnesses weren't related to a single location that would allow them to suggest a specific cause.
The Chicago Tribune reported, and a spokesperson at Rush University Medical Center later confirmed, that three people became sick overnight, suffering flu-like symptoms.
When asked about it earlier in the day, the executive director of the NATO Host Committee said she didn't have confirmed information.
"We've heard that but there are a lot of rumors," Lori Healey told NBC Chicago and other reporters.
The Tribune reported that one person was released early Monday from Rush University Medical Center. Others were expected to be released Monday afternoon.
Officials are investigating the possibility of food poisoning.