Expectant mothers are to be transferred from the Belfast Royal Hospital's maternity ward after the deaths of three babies.
It is thought that the deaths were caused by the bacterium pseudomonas aeruginosa, which is commonly found in animals and soil.
It is particularly harmful to very young people or those who are already ill. There were 25 babies in the unit, which specialises in sick and newborn babies, and they will all be moved elsewhere.
Staff launched an investigation following the deaths of two premature babies over the last week. The third death came on Friday morning.
Northern Ireland health minister Edwin Poots has confirmed that expectant mothers would be transferred to other hospitals while the investigation took place.
"This is a serious incident. The priority is to identify the source of the infection and minimise the risk of spread to other extremely vulnerable babies in the unit," he said.
Consultant Clifford Mayes, who works in the baby unit, told the BBC that the issue was only affecting the hospital's neo-natal unit.
He said the bacteria can be carried on a person's skin without them being affected and it can survive in moist conditions.
The ward was undergoing a deep clean and Dr Richard Wright, associate medical director at the Belfast Health and Social Care Trust, said it was engaged in a "fairly major investigation" into the infection.
Although the bacterium is very dangerous to an already weakened patient, there are several antibiotics that can be used to treat it.