An outbreak of the norovirus has forced the Royal Cornwall Hospital's Trust to restrict visitors at three hospitals.
Bays on seven wards at Treliske in Truro have been affected by the vomiting and diarrhoea bug.
Visitors will only be allowed for patients who are critically ill at Treliske, West Cornwall Hospital in Penzance or St Michael's in Hayle.
The trust said the bug can be picked up without direct contact and spreads easily.
The symptoms of the norovirus infection usually begin around 12 to 48 hours after a person has become infected.
It starts with a sudden onset of nausea and is followed by vomiting and diarrhoea.
Outpatients clinics will continue to run but elective surgery is being reviewed on a daily basis.
Urgent surgery will go ahead as planned.
The director of nursing, Christine Rashleigh, said: "The trust is taking a number of containment measures to try to prevent the spread of norovirus.
"We are asking people to help us by not visiting our hospitals at this time."