Cardiff and Vale health board said admissions to the University Hospital of Wales will go to a different ward or a different hospital.
The board has not confirmed what the infections are.
Babies already being treated on the ward will remain there, so it has not closed completely.
Ruth Walker, the health board's executive nurse director, said: "In order to protect very vulnerable babies we will not be taking any planned or routine admissions over the coming days."

The jury was unable to reach a verdict after 30 hours of deliberation in charges of murder and voluntary manslaughter against Ray Tensing, 27.
The Cincinnati officer killed Sam DuBose, 43, after pulling him over on 19 July 2015.
A prosecutor called it the "most asinine act" ever by a police officer.
Judge Leslie Ghiz declared the mistrial on Friday after the jury of nine white and three black people were unable to reach a verdict.
The jurors told the judge they were almost evenly split.
It is the fourth high-profile failure recently to convict a police officer in the death of a black man:
Mr Tensing stopped Mr DuBose near campus for a missing front licence plate, and police body cam shows their initial exchange was friendly.
The officer asks for Mr DuBose's driver's licence, but he says he does not have it. He also shows an unopened bottle of alcohol in the car.
Mr Tensing then asks Mr DuBose to unbuckle his seatbelt. Almost immediately after that, a shot is fired and the car appears to move.
The officer fired a single round, hitting Mr DuBose in the head.
Mr Tensing had testified that he feared he could be run over as Mr DuBose tried to drive away.
But an expert said his frame-by-frame analysis of Mr Tensing's body camera video showed he was not being dragged by the car.
Donyetta Bailey, who is president of the Black Lawyer's Association of Cincinnati, said the Tensing case and others underline that juries are "implicitly biased" in favour of police officers.
"It shows our legal system has no value for African-American human rights," said Ms Bailey, reports the Cincinnati Enquirer.
Mr Tensing was fired last year by the University of Cincinnati, which announced an overhaul of its policing department.
The university also reached a $5.3m (Â£4.1m) settlement with Mr DuBose's family, including free undergraduate tuition for his 13 children.
Mr Tensing's first trial last year also ended with the jury deadlocked.
The neonatal intensive care unit at a Cardiff hospital has stopped taking admissions due to a "cluster of infections".
[NEXT_CONCEPT]
A mistrial has been declared for the second time in the case of a white police officer who shot dead an unarmed black motorist in Ohio two years ago.