Four wards have been closed at the Royal United Hospital (RUH) in Bath due to an outbreak of Norovirus.
No further patients will be admitted to those wards until 72 hours after patients are deemed free of infection.
RUH bosses are asking people not to come to the hospital unless the visit is essential.
The diarrhoea and vomiting bug is highly infectious and the hospital wants to prevent it spreading. Outpatients clinics are open as usual.