Chicago Teachers Union President Karen Lewis is suffering from an undisclosed “serious illness” and will step aside as head of the organization, the union’s vice president announced Thursday.

But there’s still no word on how that might affect a possible mayoral run against Rahm Emanuel.

At a press conference late Thursday afternoon, Vice President Jesse Sharkey announced that Lewis underwent a successful surgery on Wednesday, but declined to name Lewis’ condition, citing her family’s privacy.

Lewis, 61, has been seriously considering a run for mayor. Sharkey said he will take over Lewis’ duties at the CTU, but wouldn’t get into the possible political impact of Lewis’ health.

“I understand that many people in this room and many people in the city want to know about Karen Lewis’s health status because they care about the mayoral election in this city,” Sharkey told reporters. “That’s a question that I can’t answer.”

Lewis was hospitalized Sunday night after experiencing discomfort, but the union and representatives with her exploratory campaign refused to say why or give any details on the status of her condition.

On Monday, CTU spokeswoman Stephanie Gadlin said in a statement that she was “in good spirits—and still thinking of creative ways to secure the future and city our students and their families deserve.”

On Wednesday night, a spokeswoman for Lewis’ mayoral exploratory committee declined to comment on the details of Lewis’condition, but said the “exploratory process is moving forward.”

Despite contentious relations in the past, Emanuel praised Lewis late Thursday afternoon in an emailed statement, though he steered clear of mentioning politics.

“Karen Lewis is a passionate advocate for her beliefs and has always been willing to speak up for her view of what’s best — not only for the teachers that she represents, but also for issues critical to the future of our city,” Emanuel was quoted as saying. “Along with all Chicagoans, we will keep Karen and her family in our thoughts and prayers, and we hope to see her on her feet very soon.”

Lewis has not officially announced whether she plans to challenge Emanuel in February’s city election. But there has been widespread speculation and encouragement from some progressives for her to run.

In recent weeks, the once-fiery critic of Emanuel who led Chicago teachers on their first strike in 25 years has sought to rebrand herself as a consensus-builder, holding several community events around the city dubbed “Conversations with Karen.” Lewis has also started fundraising for a possible campaign, though she has conceded it will be difficult to top Emanuel’s political machine, which has already netted him at least $8.3 million for his re-election bid.

Mayoral candidates have until Nov. 24 to file their nominating papers in order to get on the ballot for the Feb. 24 election. Emanuel already faces several declared challengers, including his vocal critic in the City Council, Ald. Bob Fioretti; Dr. Amara Enyia, an urban development consultant; former Chicago Ald. Robert Shaw; Chicago police officer Frederick Collins; and conservative activist William J. Kelly.

“She is a fighter and I know that she will bounce back, stronger than ever,” Fioretti said of Lewis in an emailed statement. “Her voice adds to the debate in Chicago and we all get better results when there is a full and spirited dialogue.  But right now, we should all respect Karen’s privacy and give her the space she needs to get better.” 

WBEZ political reporter Alex Keefe contributed to this story.