Six babies on a hospital neonatal unit in Norfolk have been affected by an outbreak of a bacterial infection which is usually seen in the community.
One very sick baby, born at 27 weeks, who had Panton-Valentine Leukocidin (PVL) staphylococcus aureus has died.
Experts stressed this was not the same bug as the PVL strain of the superbug MRSA which killed two patients at a West Midlands hospital.
The infection which the babies have can be treated with some antibiotics.
Microbiologists at Norfolk and Norwich University Hospital are now investigating if the rare PVL infection contributed to the baby's death.
None of the other five babies have an active PVL infection. All are in isolation and being treated with antibiotics.
The hospital said they are doing well.
Another baby on the 28-bed unit is being tested to see if it too is infected.
The babies were affected by a bacteria known as MSSA - methicillin-sensitive staphlococcus aureus - which is a form of the bug which can cause problems in the body, but which does respond to treatment with the antibiotic methicillin as well as other drugs.
The more well-known MRSA, is resistant to methicillin and therefore much harder to treat - which is why it has been dubbed a superbug.
The hospital is now testing other babies on the unit, their close relatives and members of staff are being tested to see if they are carrying the PVL strain.
The neonatal unit has also had specialist cleaning and has been closed to new admissions from other hospitals. Visiting is being restricted to parents only.
The source of the infection is not yet known.
However, PVL infections tend not to be hospital-based, and it is most likely to have been brought into the unit between 22 November and 19 December.
Of the five confirmed cases, two were transferred to Norwich from other East Anglian hospitals, one was a home birth, and two were born at the hospital.
Paul Forden, chief executive at the hospital, said: "Our neonatal unit cares for the very youngest and most critically ill of all our patients.
"It is tragic that this infection may have played a part in the death of a 27 week old baby and our thoughts are with the parents concerned."
Consultant microbiologist Dr Judith Richards added said the affected babies were all being successfully treated and were all well.
She added: "We believe the control measures introduced mean that these babies, and others in the unit, are not now at any significant risk from this strain of S. aureus.
"Our routine microbiology monitoring did very quickly pick up a rise in background S aureus and rapid further investigation has confirmed PVL in five babies.
"We have taken prompt action to treat the babies concerned and we are working with the Health Protection Agency to identify the source of this infection."