Thirteen hospital wards have been closed in Devon and Cornwall because of a sickness and diarrhoea bug.
In Devon, eight wards have been shut at the Royal Devon and Exeter Hospital (RDE) because of the norovirus.
Elsewhere in the county, two wards at Tiverton Hospital and one each at Torbay and Exmouth have been shut to new admissions.
One ward has been shut at the Royal Cornwall Hospital in Truro. Visitors are not allowed on any of the wards.
Anyone who has been in contact with the bug is asked to stay away.
Patients due for treatment who have had symptoms, or were close to someone who had, are asked to contact their hospital for advice.
The affected wards at the RDE include: Avon, Bolham, Bovey, Clyst, Culm, Durbin, Kenn and Okement.
Elaine Hobson, director of operations at the RDE, said: "Norovirus spreads very easily through contact with anyone who already has it.
"For hospitals it's a problem when anyone who has had the illness, or been exposed to it without their knowledge, comes in as a patient or a visitor. "Without realising it they may pass the bug to others - which is why we need to discourage all non essential visits to the hospital."
She added: "We are experiencing a high number of norovirus cases within the hospital which has prompted our closure of wards.
"Despite the publicity and notices throughout the hospital site, including the entrance to affected wards, people are still coming in to see their relatives and friends which could compromise our infection control measures and put their own well-being at risk."
Visitors will only be allowed access to the closed wards in "exceptional circumstances" and only when the visit has been agreed in advance with the matron.