Lewis, a former chemistry teacher at King College Prep, was diagnosed with brain cancer three years ago, forcing her to scuttle plans to challenge Mayor Rahm Emanuel's bid for re-election.
"I suffered a slight stroke last week and have since been hospitalized," a statement released by Lewis said. "It was so minor that doctors could barely see it upon my initial examination, so please know that my spirits are high, and I am feeling pretty good. This is a minor setback in my recovery from brain cancer, but rest assured I will be back on my feet soon."
In a statement Tuesday, Emanuel said he called the union president to wish her a "speedy recovery" and found her in "good humor."
"Karen Lewis has always been a strong voice for Chicago's children and Chicago's teachers, and the strength of her voice is only matched by the strength of her will," Emanuel said. "A stroke can't keep Karen Lewis down."
Emanuel said Tuesday afternoon he sent her some matzo ball soup from Manny’s.
Emanuel, who famously used an expletive in one of his first meetings with Lewis, praised the union president's "passion" for Chicago's students and schools.
"She has incredible strengths, and she sounds strong," Emanuel said. 'And I said 'You know, we need you back on the field here making sure that everybody stays focused, and I hope you recover soon. If you need anything, let me know.'"
Lewis thanked those who sent good wishes.
"My only request is that we refrain from talking policy and politics until I am back up to speed," Lewis said. "Once I return to 100 percent good health, I’ll be back to giving hell to the usual suspects — you know who you are.”
Lewis told NBC5 that when she woke up Thursday she could not move her left leg or her left hand. The stroke occurred in the same part of her brain that the tumor was discovered in 2014, she said.