Five wards have been temporarily closed to new admissions at Altnagelvin Hospital due to the winter vomiting bug.
The so called winter vomiting bug has also closed a ward in the Waterside Hospital.
Several other areas are being closely monitored.
A number of increased infection control measures have been put in place including the isolation of symptomatic patients, as well as the use of Contact Precaution, the use of Personal Protective Clothing, increased and enhanced cleaning and limiting the movement of staff between wards unless necessary.
A spokesperson for the Western Trust said:
"Our Infection Prevention and Control Team are closely monitoring a number of other wards and continuing to provide advice to staff.
"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. People should not come to visit unless absolutely necessary.
"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."
People are also being advised to consider whether a trip to A&E is the best option this winter, Altnagelvin Hospital recently launched a plan to help alleviate pressure on the service this winter.
For more information about Norovirus, click here: http://bit.ly/2gxmINk