His 64-year-old alleged attacker, who can not be named for legal reasons, was led out of her two-storey home at The Gap on Tuesday morning and questioned by police for hours before being charged in the afternoon.
It's understood the baby's mother was visiting the home and was woken by the piercing sounds of her child crying.
"The mother found the child in a distressed state with injuries to his face", Inspector Daniel Bragg told reporters.
A hospital spokesman says the boy undergoing surgery but his condition is not known.
The four-month-old boy was transported to the Lady Cilento Hospital where he remains in a serious condition and is now undergoing surgery.
"We're used to going to trauma but it's always probably the most affecting job you can go to, one involving a child", he said.
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'The ambulance officers who attended were visibly distressed by what they were dealing with. the police officer who was first on the scene was visibly affected by what he saw'.
Police alleged the boy's mother walked into his room and found the 64-year-old woman "in possession" of a large knife.
Police collect evidence at a home in The Gap.
Stunned neighbour Sara Gomez struggled to process the allegations levelled against the woman she had thought was a "nice lady".
"I don't know her very well but it doesn't seem like, it's very uncharacteristic - well - completely uncharacteristic", she said.
She is expected to appear at the Brisbane Magistrates Court tomorrow, October 18.