A ward at Altnagelvin Hospital in Derry has been closed to new admissions because of an outbreak of Norovirus.
Ward 42 has been temporarily closed as a result of the bug, which causes diarrhoea and vomiting.
The Western trust has urged anyone who is unwell not to visit the hospital. It has also issued guidelines to visitors surrounding hygiene.
It is the second outbreak of the Norovirus in a month at the hospital. In June, Ward 20 was closed.
The main symptoms of the Norovirus infection are sudden nausea, stomach ache followed by severe "projectile" vomiting and / or diarrhoea.
Symptoms usually begin between 12 and 48 hours after a person becomes infected.