A deep clean is being carried out at the neonatal unit in Altnagelvin Area Hospital in Derry after an increase in babies being diagnosed with a bloodstream infection.
Three babies have developed MSSA (Methicillin-sensitive Staphylococcus Aureus) since last month.
The Western Health Trust said a deep clean and thorough decontamination process is taking place, including the screening of babies.
The neonatal unit is still open, but it is not receiving transfers from other trusts.
Western Trust Medical Director Dr Anne Kilgallen said: "Since May 2013, three babies have been diagnosed with MSSA bloodstream infection.
"MSSA bloodstream infection occurs from time to time, however, it is unusual to have a cluster of three infections."
The infection occurs naturally on the skin of both babies and adults and is usually carried without ill effects. But it does pose a potential risk to a small number of babies.
No information has been given so far on the condition of the three babies.
