A ward at Altnagelvin Hospital has been closed due to an outbreak of vomiting and diarrhoea, after four patients and two staff were struck down with the sickness.
The Western Health and Social Care Trust, which runs the hospital, has asked people who are feeling unwell not to visit for fear of spreading the infection.
Anyone who has been ill should wait 72 hours before making a trip to the hospital, Director of Acute Services Geraldine McKay said.
She also urged those who do have to make a visit to wash their hands thoroughly before and after the trip.
Despite the closure of ward 8, where head and neck patients are treated, the trust said no other area of the hospital is affected, including any other appointments or services being cancelled.