Almost 300 patients and staff have now been struck down with vomiting and diarrhoea at two East Yorkshire hospitals.
Visiting hours are being restricted at Hull Royal Infirmary.
The virus has spread across 24 wards at Hull Royal Infirmary and Castle Hill Hospital in Cottingham.
Visitors have been told not to bring fruit and flowers in case they transmit the airborne virus.
Manager say the spread of the virus is starting to slow, but stress the threat of more cases is not over yet.
The illnesses have been caused by the Noro virus which is a common cause of this type of sickness.