About 100 staff and patients have fallen ill with sickness and diarrhoea at two hospitals in East Yorkshire.
A total of 14 wards have been affected at Hull Royal Infirmary and Castle Hill Hospital in Cottingham.
Some routine operations have been cancelled and managers are asking visitors to stay away from the hospitals while wards are disinfected.
Gifts of flowers and fruit which have already been brought on to wards are being thrown away.
The illnesses have been caused by the Noro virus which is a common cause of this type of sickness.
Doctors are stressing that urgent surgery is still taking place.