Fighting Pneumonia in Hospital ...

Nick Cannon just had a major health SCARE ... landing him in the hospital with pneumonia.

The "Masked Singer" host posted a photo of himself in a hospital bed, wearing a face mask, gown and beanie.

The father who has welcomed 11 kids with 2 on the way casually revealed his diagnosis to his millions of IG fans ... "Its just pneumonia, nothing I cant handle. Okay, so I guess Im not Superman."

Then Nick became more serious, making a veiled reference to his 10-year battle with the autoimmune disease lupus ... "I promised myself I would never be back at this place again… great lesson to take care of YOU or YOU wont be able to take care of everyone else."

Nick said he didnt want prayers or well wishes...he just needs some much-needed rest and hell be back "stronger than ever."

Whats more ... Nick is currently on his "Cannon Wild N Out" tour and performed a sold-out show Thursday night at Madison Square Garden in NYC. He reflected, "Now Im all alone in a tiny hospital room. Life is definitely a rollercoaster!"

Feel better, Nick!!