A number of wards at Altnagelvin Hospital in Derry have been closed due to an outbreak of the norovirus vomiting bug.
Admissions to the affected wards have been temporarily suspended while a number of others are being “closely monitored”.
The Western Health Trust said infection control measures had been put in place to stop the virus spreading.
Norovirus, also known as the winter vomiting bug, is present all year but becomes more common over winter months.
Patients with symptoms have been put in isolation and movement between wards 7, 31, 32 and 40 is now limited.
“To protect our patients and staff, people should not come to the hospital if they are feeling unwell, have any symptoms of norovirus or someone at home does,” the Western Trust said.
“Those who decide to visit loved ones in the hospital are asked to thoroughly wash their hands before and after visiting.
“Visitors should visit only one patient whilst at the hospital, refrain from sitting on hospital beds and not move from ward to ward when visiting.”