A number of patients are ill with vomiting and diarrhoea at a third ward at King's Mill Hospital in Nottinghamshire.
The hospital closed two wards on 4 November after a norovirus outbreak.
The third ward remains open but anyone with norovirus symptoms is being asked to stay away from the hospital.
All affected wards would be deep cleaned and reopened when the patients have recovered, the hospital said.
Norovirus, also known as the winter vomiting virus, is very easily spread from person to person.
The virus lasts around two days and no treatment is required, but even after the symptoms have cleared up people may still carry the virus and infect others up to three days after their own symptoms have stopped.
There are currently 14 patients and six members of staff affected at the hospital.
Noroviruses affect up to one million people a year in England and Wales.