Mary Lou Retton Suffers 'Scary Setback,' Remains In ICU

Mary Lou Retton has suffered a "scary setback" this week in her battle with a rare form of pneumonia ... and she now remains in the ICU fighting for her life.

The Olympic legend's daughter, Shayla, provided the concerning update in a video on her social media page Wednesday evening ... saying Retton had been showing signs of improvement prior to the turn.

"Basically, at the beginning of this week," Shayla said, "we were going on the up-and-up. We were so excited. Seeing so much progress. And then yesterday, we had a pretty scary setback. She is still in ICU."

Thankfully, there is at least a bit of positive news ... Shayla said Retton had a much better day Wednesday, though the 55-year-old former gymnast is clearly nowhere near out of the woods yet.

"We're just working through some things as far as her setback goes," Shayla said.

Retton has been in the ICU since earlier this month ... though plenty have sent prayers her way in the hopes she improves. So far, over 8,000 donors have given more than $450,000 to a SpotFund created on her behalf ... something Shayla said she's super grateful for.