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The Xtra Factor co-host suffered an allergic reaction at her hotel in Liverpool just hours before the first filmed X Factor auditions in the city, reports The Sun.
She was said to have been violently sick and became feverish before being treated by medics at the hospital.
"Caroline was delirious. She had no idea what was wrong with her," a source said.
"She feared she would have to miss the first show."
However, she was later sent back to the hotel and was able to attend the X Factor auditions at Liverpool's Echo Arena.
Flack later tweeted: "After a VERY long night we are up and ready to roll! First auditions in Liverpool... and a new judge."
Caroline Flack was officially confirmed to be returning as co-host of The Xtra Factor with Olly Murs earlier this week.
Picture gallery - X Factor first auditions:
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