Two wards at Enniskillen's Erne Hospital were closed to new admissions on Thursday after a suspected outbreak of the Norovirus bug.
The move followed an increase in vomiting and diarrhoea among patients in a number of wards, particularly those with elderly patients.
One of the wards has since re-opened but the other will remain closed until at least Friday.
Anyone who is feeling unwell has been advised to avoid visiting the hospital.
The Norovirus is common in the community at this time of year and is a short-term, unpleasant illness, with most people getting better within a few days.