Three wards at the Royal Cornwall Hospital at Treliske have been closed to new admissions following an outbreak of a stomach bug. It is not yet known if the infection is the Norwalk virus, but several patients are unwell with diarrhoea and vomiting.
People are being asked not to visit friends and relatives if anyone at home has similar symptoms.
In December, Plymouth's Derriford Hospital closed six wards after a similar virus.