The winter vomiting bug Norovirus has broken out at Poole Hospital.
Seven wards are closed to new admissions due to the bug and staff have advised people to check visiting times.
A spokesman said patients should attend for outpatient appointments and tests as normal unless they have had the sickness bug in the past three days.
No one should visit the hospital if they have had any sickness or diarrhoea symptoms in the last 72 hours.
Denise Richards, infection control matron, said: "Members of the public should be aware that Norovirus is circulating in the community and they should be alert to the symptoms.
"Certainly if anyone has had any diarrhoea or sickness they should not visit the hospital until they have been clear of these symptoms for three days."