The hospital has identified 12 listeria infections since Oct. 1, including the fatalities that hospital officials said were likely facilitated by the patients' weakened immune systems from previously contracted illnesses, Aftenposten reported Friday.

Hospital officials said the source of the bacteria has not yet been pinpointed, but they said it may have come from contaminated cold food, possibly cheese.

Listeria bacteria has been known to cause listeriosis, a serious illness with a mortality rate of 25 percent. Pregnant women, newborns and adults with weakened immune systems are especially susceptible to the illness.

All refrigerated food at the hospital has been discarded, but officials said new cases may still arise as the bacteria's incubation period can last for up to a month.